r 


REESE  LIBRARY 
UNIVERSITY  OF  CALIFORNIA. 


No.  wD  V /  f. 


THE 


DEVELOPMENT 


AND 


PRESENT   ASPECTS 


OF 


STEREO-CHEMISTRY 


BY 

CHARLOTTE   F.  ROBERTS,  PH.D. 


BOSTON,   U.S.A. 
D.  C.  HEATH    &   CO.,  PUBLISHERS. 

1896. 


v, 


COPYRIGHT,  1896, 
BY  CHARLOTTE  F.  ROBERTS 


&  r; 

P^p 

JOHN  WILSON  AND  SON,  CAMBRIDGE,  U.S.A. 


•    *> 

i 


CONTENTS. 


PAGE 

I.     GENERAL  PRINCIPLES  OF  STEREO-CHEMISTRY  ...         1 
II.     ILLUSTRATIONS  AND    APPLICATIONS   TO   PARTICULAR 

CASES 37 

III.  THE  BENZENE  SERIES    v  .'  * 68 

IV.  THE  STEREO-CHEMISTRY  OP  NITROGEN 105 

V.     VARIATIONS   IN   OPTICAL   ACTIVITY  AND   RELATIONS 

OF  STEREO-CHEMISTRY  TO  CRYSTALLOGRAPHY  .     .     138 
VI.     DEDUCTIONS    AND    SPECULATIONS    CONCERNING    THE 
NATURE  OF  ATOMS  AND  VALENCE,  WHICH  HAVE 

GROWN     OUT      OF     THE     STUDY     OF     STEREO-CHEM- 
ISTRY      .  153 


LIST  OF  BOOKS  CONSULTED 190 

PERIODICALS  TO  WHICH  REFERENCE  HAS  BEEN  MADE     191 


Of  THE 

(•UNIVERSITY 


STEREO-CHEMISTRY. 


i. 

GENERAL  PRINCIPLES   OF  STEREO-CHEMISTRY. 

SINCE  the  introduction  of  the  Atomic  Theory,  great 
advances  have  already  been  made  in  the  definiteness  of  our 
conceptions  in  regard  to  the  internal  structure  of  molecules, 
and  it  is  largely  to  the  study  of  the  phenomenon  of  isomerism 
that  this  growth  is  due.  There  has  been  a  gradual  evolution 
of  ideas  from  the  vague  conception  of  matter  as  continuous , 
to  the  idea  of  matter  as  made  up  of  definite  molecules,  sepa- 
rate and  distinct,  then  of  these  molecules  as  made  up  of  a 
definite  number  of  separate  and  distinct  atoms,  and  finally, 
through  the  study  of  isomerism,  to  the  idea  of  these  atoms  as 
possessing  a  definite  arrangement  in  the  molecule,  one  atom 
being  directly  joined  to  another,  forming  a  kind  of  chain.  It 
is  this  last  conception  which  has  given  us  our  ordinary  struc- 
tural formulas ;  and  we  must  certainly  feel  that  it  is  a  great 
triumph  of  human  ingenuity  when  we  are  enabled  to  deter- 
mine the  method  of  atomic  linking  from  a  study  of  the  chem- 
ical reactions  of  the  body  in  question. 

But  the  study  of  isomerism  leads  us  a  step  further,  and 
gives  us  a  glimpse,  at  least,  of  the  possible  arrangement  of 
the  atoms  in  space,  and  the  geometrical  forms  which  groups 
of  combined  atoms  assume.  This  branch  of  science,  known 


2  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

variously  as  the  study  of  "  Geometrical  Isomerism,"  or  "  Stereo- 
Chemistry,"  or  "Chemistry  in  Space,"  has  been  almost 
entirely  developed  within  the  last  twenty  years,  but  the 
literature  of  the  subject  scattered  through  the  various  scien- 
tific journals  is  already  enormous,  and  the  underlying  ideas 
in  some  form  or  other  are  now  accepted  by  most  eminent  stu- 
dents of  Organic  Chemistry. 

The  foundation  for  much  that  is  now  accepted  in  stereo- 
chemical  theories  was  laid  by  Pasteur  in  1860-61.  In  his 
"B-echerches  sur  la  dissymetrie  moleculaire  des  produits 
organiques  naturels,"  he  referred  the  peculiar  optical  isome- 
rism  of  the  tartaric  acids  to  a  lack  of  symmetry  in  the  mole- 
cule, just  as  optical  activity  in  crystals  is  referred  to  a  lack 
of  symmetry  in  the  crystalline  individual." 

In  1873  Wislicenus  gave  a  new  impulse  to  the  discussion  of 
these  problems  by  pointing  out  the  inadequacy  of  structural 
formulas  to  explain  the  isomerism  in  the  lactic  acids.  In 
his  investigation  of  these  acids,  he  found  that  ordinary  and 
sarco-lactic  acids  must  be  represented  by  the  same  structural 
formula,  and  yet  they  were  undoubtedly  two  distinct  bodies, 
one  being  active  as  regards  polarized  light,  the  other  inactive. 
He  was  thus  led  to  the  conclusion  that,  since  the  atomic  link- 
ing must  be  the  same  in  both,  the  difference  in  physical' 
properties  could  only  be  accounted  for  on  the  supposition  of^/ 
a  different  relative  arrangement  of  the  atoms  in  space.  This*" 
suggestion,  in  van't  Hoff's  own  words,  set  him  to  thinking, 
and  to  such  good  purpose  that  the  results  of  his  thinking  are 
now  accepted,  with  some  modifications,  by  most  students  of 
Organic  Chemistry. 

In  van't  Hoff's  work,  as  is  well  known,  the  organic  com- 
pound is  symbolized  by  a  tetrahedron,  with  the  carbon  atom 
situated  in  the  centre,  and  the  combining  atoms  or  radicals  in 


GENERAL  PRINCIPLES   OF   STEREO -CHEMISTRY.  3 

the  solid  angles.  It  should  be  mentioned  that  the  tetrahedral 
symbol  was  suggested  by  Kekuld  as  early  as  1867,  as  repre- 
senting well  the  quadrivalence  of  carbon,  and  the  equal  value 
of  the  four  bonds.  Before  entering  into  the  details  of  van't 
Hoff's  work,  it  may  be  advantageous  to  review  some  simple 
cases  in  which  the  use  of  this  symbol  is  shown  to  be  an 
improvement  over  the  ordinary  structural  formula  represent- 
ing the  molecule  in  a  plane,  but  it  should  be  remembered 
that  any  molecular  formula  must  be  looked  upon  as  a  symbol 
expressive  of  thoroughly  ascertained  facts  and  not  as  a  com- 
plete picture  of  the  molecule,  but  that  that  symbol  will  be 
the  best  which  illustrates  the  largest  number  of  facts,  and  is 
out  of  harmony  with  none. 

At  the  outset,  we  must  assume  that  it  is  as  essentiaj  that 
the  atoms^^^omposing^a  molecule  should  -arrange  themselves 
in  some  definite  position  of  equilibrium,  depending  on  their 
affinity  for  each  other,  as  that  the  planets  of  the  solar  system 
should  retain  their  relative  positions.  Without  external 
force  or  some  new  disturbing  influence,  the  relative  positions 
can  no  more  be  changed  in  one  case  than  in  the  other.  If, 
for  example,  we  consider  the  molecule  CH4,  it  is  but  reason- 
able to  suppose  that  the  atoms  will  arrange  themselves  in 
such  a  way  that  all  of  the  hydrogen  atoms  will  be  at  the 
same  distance  from  the  carbon,  and  also  that  each  hydrogen 
atom  will  be  equidistant  from  all  of  the  others.  This  con- 
dition of  affairs  is  satisfied  by  the  conception  of  the  carbon 
atom  situated  in  the  centre  of  a  tetrahedron  which  has  a 
hydrogen  atom  in  each  of  its  four  solid  angles. 

Again,  consider  the  molecule  CH3C1.  There  can  be  no 
reason  for  supposing  that  one  hydrogen  atom  is  differently 
situated  in  relation  to  the  chlorine  from  the  others,  and 
there  is  nothing  in  the  chemical  behavior  of  the  body  to 


4  THE  DEVELOPMENT    OF   STEREO-CHEMISTRY. 

suggest  such  an  arrangement ;  yet  this  is  exactly  what  is 
suggested  by  the  ordinary  plane  formula,  — 

H 

H  -  C  -  Cl 
H 

To  show  this  body  in  a  position  of  stable  equilibrium  we 
should,  rather,  picture  the  three  hydrogen  atoms  all  at  equal 
distances  from  the  carbon,  and  all  at  equal  distances  from 
the  chlorine,  and  this  condition  of  affairs  would  be  perfectly 
represented  by  a  tetrahedron  with  the  carbon  atom  in  the 
centre,  and  the  hydrogen  and  chlorine  in  the  solid  angles. 

As  another  illustration,  suppose  two  of  the  hydrogen  atoms 
in  CH4  to  be  replaced  by  chlorine,  giving  CH2C12 ;  using  the 
ordinary  plane  formulas,  this  could  be  represented  in  two 
ways :  — 

H  H 

I  I 

Cl  _  C  -  H        or        Cl  -  C  -  Cl 

I  I 

Cl  H 

These  two  formulas  should  represent  two  different  isomeric 
bodies,  but  no  such  isomerism  has  ever  been  observed.  But 
if  we  consider  the  molecule  as  a  system  held  in  equilibrium 
by  the  mutual  attraction  and  interaction  of  all  of  its  parts, 
even  the  atoms  which  are  not  directly  linked  exerting  some 
force  of  attraction,  then  it  is  plain  that  there  will  be  only 
one  position  of  the  atoms  in  which  they  will  be  in  a  state  of 
stable  equilibrium,  and  that  the  arrangement  will  be  a  sym- 
metrical one,  corresponding  again  to  the  tetrahedral  figure, 
with  the  hydrogen  and  chlorine  atoms  in  the  solid  angles. 
It  is  not,  however,  necessary  to  suppose  that  the  form  of 


GENERAL  PRINCIPLES  OF   STEREO-CHEMISTRY.  5 

the  molecule  is  always  that  of  a  regular  tetrahedron,  with 
the  combining  atoms  or  radicals  fixed  in  the  solid  angles. 
Without  at  present  discussing  the  question  of  oscillatory  mo- 
tions of  the  atoms  in  the  molecule,  the  mean  form  assumed 
by  a  carbon  atom  with  its  four  attendant  radicals  would  prob- 
ably be  that  of  a  regular  tetrahedron,  in  case  the  four  atoms 
or  radicals  were  alike ;  but  if  different,  some  may  be  drawn 
nearer  to,  and  others  driven  farther  from,  the  carbon,  thus 
forming  irregular  tetrahedrons. 

The  illustrations  given  above  will  perhaps  suffice  to  show 
that  the  tetrahedral  symbol  of  carbon  is  in  more  complete 
accordance  with  observed  facts  than  the  ordinary  formulas 
in  which  the  molecule  is  represented  in  a  plane,  but  the  prin- 
cipal use  of  the  tetrahedral  formula  has  been  in  explaining 
cases  of  physical  isomerism.  Such  cases  are  too  well  known 
to  need  illustration,  but  the  phenomenon  is  so  well  shown  by 
the  tartaric  acids  that  a  brief  allusion  to  these  must  be  par- 
doned. It  is  well  known  that  ordinary  tartaric  acid  is  dextro- 
rotatory. Kacemic  acid,  which  resembles  tartaric  acid  so 
closely  that  it  must  be  assigned  the  same  structural  formula, 
differs  from  it  in  its  crystalline  form  and  optical  properties, 
racemic  acid  being  optically  inactive.  Now  if  this  latter  acid 
in  the  form  of  its  sodium-ammonium  salt  be  allowed  to  evapo- 
rate spontaneously,  crystals  of  two  kinds  are  developed,  as 
shown  in  the  following  figures :  — 


>i 


THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 

In  each  kind  there  is  a  hemihedral  development,  and  each  is 
the  complementary  form  of  the  other,  so  that  a  crystal  of  one 
kind  held  before  a  mirror  gives  an  image  which  is  exactly 
like  a  crystal  of  the  other  kind.  One  of  these  kinds  of 
crystals  is  found  to  be  ordinary  dextro-tartaric_.acid,  while 
the  oppositely  developed  crystals  rotate  the  plane  of  polari- 
zation to  the  left,  and  are  laevo-tartaric  acid!  Racemic  acid 

is    thus    proved   to   be    a    mixture    of    twj">    physif.nl    isnmprirlps 

which  rotate  the  plane  of  polarization  equally  in  opposite 
directions;  and  in  general  it  may  be  said  that  the  existence 
of  any  optically  active  body  implies  the  possible  existence  of 
another  which  will  rotate  the  plane  of  polarization  in  the 
opposite  direction,  and  which,  if  the  bodies  crystallize,  will 
be  ah  enantiomorphous  form. 

It  has,  of  course,  been  long  known  that  many  crystalline 
bodies  rotate  the  plane  of  polarization  of  light;  but  the 
peculiarity  in  the  case  of  the  active  organic  bodies  is  that 
their  solutions  have  this  power,  and  in  some  cases  it  has  also 
been  observed  when  the  substance  was  in  the  state  of  vapor. 
The  phenomenon  in  these  cases,  then,  must  be  ascribed  to 
some  property  inherent  in  the  molecule.  In  crystals  this 
optical  activity  has  long  been  attributed  to  a  certain  lack  of 
symmetry  in  the  arrangement  of  the  molecules  of  the  crystal, 
since  it  has  been  observed  only  in  bodies  which  show  a  hemi- 
hedral or  tetartohedral  development,  and  which  occur  in 
enantiomorphous  forms.  It  is  natural  to  infer  that  optical 
activity  would  in  all  cases  be  produced  by  similar  causes.  It 
follows,  then,  that  in  those  organic  compounds  which  are 
optically  active,  there  must  be  a  lack  of  symmetry  inside  the 
molecule  itself.  JNow,  going  back  to  the  tetrahedral  symbol, 
if  we  have  an  asymmetric  carbon  atom,  • —  that  is,  a  carbon 
atom  joined  to  four  different  atoms  or  radicals,  —  we  have  a 


GENERAL  PRINCIPLES   OF   STEREO-CHEMISTRY.  7 

tetrahedron  which  has  no  plane  of  symmetry.  In  such  a  case 
ns  tliis  ind  onlv  in  snch  a  case  is  it  uossible  still  usin0"  the 
tetrahedral  symbol,  for  the  atoms  to  have  two  different  posi- 
tions with  regard  to  the  carbon,  the  relation  between  the  two 


figures  being-  |jfra.f.  pf  an  ^j^f  "^'i  its  reflected  image,  or  that 
between  two  enantiombrphous  crystalline  forms. 

In  the  accompanying  figures,  in  relation  to  the  carbon  atom 
situated  at  the  centre,  the  order  a  b  c  in  (1)  is  to  the  right, 
and  in  (2)  to  the  left,  and  any  three  of  the  groups  will  be 
found  similarly  to  lie  in  reverse  orders  in  the  two  forms. 


We  thus  carry  over  into  the  molecule  the  idea  of  righfc- 
handed  and  left-handed  forms  similar  to  the  forms  which 
have  been  observed  in  crystals  which  have  a  similar  effect 
on  polarized  light.  The  bodies  are  non-superposable.  Their 
close  similarity,  their  identity  of  atomic  linking,  forbids  that 
there  should  be  any  difference  in  their  behavior  toward 
reagents,  but  allows  slight  physical  differences,  prominent 
among  these  being  that  if  one  rotates  the  plane  of  polariza- 
tion to  the  right,  its  enantiomorphous  form  must  rotate  it  to 
a  corresponding  degree  to  the  left.  According  to  this,  the 
existence  of  a  dextro-rotatory  body  always  implies  the  pos- 
sible existence  of  a  laevo-rotatory.  *  The  central  pivot,  then,  ^\ 
of  the  van't  Hoff-Le  Bel  hypothesis,  is  that  every  optically 
active  body  contains  an  asymmetric  carbon  atom. 


8  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

It  should  be  noted  that,  though  this  is  spoken  of  as  the 
van't  Hoff-Le  Bel  hypothesis,  the  subject  has  been  approached 
thus  far  rather  from  the  standpoint  of  van't  Hoff,  than  from 
that  of  Le  Bel.  The  latter,  whose  original  paper 1  appeared 
in  September,  1874,  two  months  before  that  of  van't  Hoff, 
bases  his  reasoning  upon  the  work  of  Pasteur  and  others,  who 
had  completely  established  the  correlation  existing  between 
molecular  dissymmetry  and  rotatory  power.  He  concludes  that 
in  any  body  of  the  type  MA4,  if  three  of  the  atoms  or  radicals 
be  replaced  by  three  different  atoms  or  radicals,  the  result 
will  be  an  unsymmetrical  body,  and  this  will  possess  rotatory 
power  provided  the  four  radicals  have  fixed  position  and  are 
not  in  one  plane.  He  therefore  makes  his  statement  the 
converse  of  van't  Hoff's,  but  agrees  with  the  latter  in  con- 
necting optical  activity,  in  general,  with  the  presence  of 
asymmetric  carbon,  though  his  reasoning  is  entirely  inde- 
pendent of  any  hypothesis  in  regard  to  the  tetrahedral  form. 
In  a  later  paper 2  he  says,  "  The  reason  which  led  me  to  give 
to  my  demonstrations  a  particular  and  less  simple  form  was 
that  I  had  doubts  as  to  whether  CB4  had  really  the  form  of  a 
regular  tetrahedron  or  not,"  these  doubts  being  based  upon 
the  observed  crystalline  form  of  some  bodies  of  the  CR4 
type. 

The  next  question  that  naturally  arises  is,  how  has  the 
van't  Hoff-Le  Bel  hypothesis  been  borne  out  by  facts?  At 
first,  succinic  acid,  styrol,  and  a  number  of  other  compounds 
were  cited  as  bodies  which  were  optically  active  and  yet  con- 
tained no  asymmetric  carbon,  but  by  careful  experiments  it 
has  been  found  that  if  these  substances  are  perfectly  pure, 
they  are  entirely  inactive,  and  at  present  no  optically  active 

1  Bull.  Soc.  Chem.  (2)  22,337. 

2  Bull.  Soc.  Chem.  (3)  3-788. 


GENERAL  PRINCIPLES   OF  STEREO-CHEMISTRY.  9 

organic  compound  has  been  discovered  whose  formula  does 
not  contain  an  asymmetric  carbon  atom.  Further,  the  deriv- 
atives of  an  optically  active  body  are  sometimes  active  and 
sometimes  inactive.  In  all  cases  in  which  the  activity  is 
destroyed,  the  carbon  atom  loses  its  asymmetry ;  in  all  other 
cases  the  formula  still  contains  asymmetric  carbon. 

It  is  to  be  noticed  that  the  converse  does  not  necessarily 
hold  good;  namely,  that  every  substance  containing  an 
asymmetric  carbon  shows  optical  activity.  Many  bodies  are 
known  whose  formulas  contain  asymmetric  carbon,  and  yet  in 
which  no  optical  activity  has  been  observed.  In  some  cases 
we  cannot  assign  any  definite  cause  for  the  lack  of  activity. 
It  may  be  that  the  rotatory  power  is  so  feeble  as  to  elude 
detection ,  or  that  the  body  is  not  sufficiently  soluble  to  show 
the  phenomenon;  but  in  many  cases  this  inactivity  can  be 
easily  explained.  In  some  chemical  reactions  there  is  noth- 
ing to  determine  whether  the  right  or  the  left  handed  body 
shall  be  formed;  one  can  be  prepared  as  readily  as  the  other. 
In  such  a  case,  the  chances  are  that  just  as  many  molecules 
of  one  will  be  formed  as  the  other,  and  the  result  will  be  a 
mixture  of  two  equally  but  oppositely  active  bodies,  which 
will  therefore  be  inactive.  Referring  to  the  illustration 
given  above,  racemic  acid  seems  to  be  a  mixture  of  dextro 
and  laevo  tartaric  acids. 

It  is  an  interesting  fact  that  when  a  compound  with  an 
asymmetric  carbon  atom  is  produced  in  a  vegetable  or  animal 
organism,  it  is  found  almost  without  exception  to  possess 
optical  activity,  whereas  when  the  same  compound  is  formed 
artificially,  the  optical  activity  is  generally  wanting.  In  the 
latter  case  it  is  probable  that  equal  quantities  of  the  right 
and  left  handed  modifications  are  formed,  producing  inac- 
tivity, but  living  organisms  have  a  tendency  to  destroy  one 


10  THE   DEVELOPMENT  OF    STEREO-CHEMISTRY. 

of  the  modifications,  so  only  one  form,  and  that  active,  is 
produced  by  their  agency. 

Many  inactive  bodies  have  been  proved,  to  be  mixtures  of 
their  dextro  and  laev^isomers  ,  and  by  the  process  called 
inesotomism  have  been  separated  into  their  active  compo- 
nents. At  the  presenT^trnrcTTlo'iriactlve  body  containing  only 
one  asymmetric  carbon  atom  has  withstood  mesotomism,  and 
on  the  other  hand,  no  inactive  body  which  does  not  contain 
asymmetric  carbon  has  been  found  capable  of  mesotomism. 

The  methods  used  to  bring  about  this  separation  are 
principally  :  — 

of  micro- 


organisms. 

These  have  unequal  aptitudes  for  destruction  of  the  two 
isomers,  and,  in  many  cases,  their  growth,  if  carried  on  long 

enough,  destroys  one  and  leaves  the  other  active  component 

p  .1  «*-^^x*»^Vx^**r 

)f  the  inactive  body. 

/'  2.  The  method  based  on  unequal  aptitude  of  the  two  active 
Jisoiners  to  combine  with  a  compound  possessed  of  rotatory 
power.  Thus  an  active  base  is  added  to  separate  the  con- 
stituents of  an  acid  or  an  active  acid  to  separate  the  con- 
stituents of  a  base.  If  inactive  malic  acid,  for  example,  is 
lalf  neutralized  by  the  active  base  cinchonine  and  allowed  to 
crystallize  in  presence  of  active  malate  of  ciuchonine,  the 
active  malate  is  separated  out. 

3.  The  method  of  crystallization  in  hemihedral,  enantio- 
morphous  forms,  already  illustrated  in  the  formation  of  the 
two  active  tartaric  acids  by  the  crystallization  of  the  sodium- 
ammonium  salt  of  racemic  acid.  This  method  can  be  used 
only  in  very  rare  cases,  since  few  of  the  organic  bodies  show 
a  sufficient  tendency  to  perfect  crystallization. 

The  second  method  given  is  evidently  applicable  only  to 


GENERAL  PRINCIPLES  OF   STEREO-CHEMISTRY.         11 

acids  and  bases,  therefore  the  method  by  use  of  micro-organ- 
isms is  the  most  general  of  the  three.  This  was  the  first  one 
used,  and  since  it  was  for  some  time  the  only  way  known  for 
producing  optically  active  bodies,  it  gave  rise  to  the  belief 
that  such  bodies  could  only  be  produced  by  the  agency  of 
life. 

Since  the  number  of  inactive  mixtures  is  large,  and  the 
methods  for  separating  them  few,  it  is  not  strange  that  there 
are  still  many  such  bodies  from  which  no  active  isomers 
have  been  obtained.  This,  however,  offers  no  proof  that 
they  are  not  really  fixtures  of  active  components,  and  the 
number  of  bodies  proved  to  be  such  mixtures  is  constantly 
increasing. 

It  has  been  suggested  by  Victor  Meyer  that  these  mesotomic 
bodies,  instead  of  being  mixtures,  may  be  polymeric  forms  of 
the  active  constituents.  In  that  case,  they  would  seem  to  be 
connected  very  closely  with  the  bodies  next  to  be  mentioned, 
—  the  amesotomic  Jnactive  bodies,  containing  asymmetric 
carbon,  but  having  a  symmetrical  formula.  In  these  cases 
there  are  at  least  two  asymmetric  carbon  atoms,  and  the  two 
halves  of  the  molecule  are  exactly  alike,  so  that  we  may  con- 
ceive of  the  atoms  as  arranged  around  one  of  the  asymmetric 
carbons  in  such  a  way  as  to  rotate  a  beam  of  light  to  the 
right,  and  arranged  around  the  other  in  the  reverse  order  so 
as  to  rotate  it  equally  to  the  left.  The  conditions,  then,  are 
similar  to  those  of  the  mesotomic  type  mentioned  above, 
except  that  the  mixture  is  inside  the  molecule,  and  the 
constituents  cannot  be  separated  from  one  another  without 
breaking  up  the  molecule.  As  an  illustration  of  an  inactive 
amesotomic  body,  we  have  inactive  tartaric  acid,  which  differs 
from  racemic  acid  in  that  it  cannot  be  separated  into  the 
dextro  and  laevo-acids.  Thus,  in  the  following  formulas  if 


12 


THE    DEVELOPMENT   OF  STEREO-CHEMISTRY. 


(1)  represents  dextro-tartaric  acid,  laevo-tartaric  acid  will  be 
represented  by  (2),  and  the  inactive  modification  by  (3). 


COOH 


OH 


OH 


The  mere  presence  of  an  asymmetric  carbon  atom  does  not 
seem  to  be  sufficient  of  itself  to  produce  an  appreciable  rota- 
tory power.  A  certain  amount  of  complexity  in  the  molecule 
seems  also  to  be  necessary,  sarco-lactic  acid, 


CH3 


OH 


\ 


•C 


\ 


COOH 


H 


being  the  simplest  substance  which  in  the  liquid  state  rotates 
the  plane  of  polarization.  Of  substances  containing  less  than 
three  carbon  atoms,  not  one  has  been  proven  active,  though 
many  contain  asymmetric  carbon.  Perhaps  some  connection 
may  be  traced  between  this  fact  and  the  work  of  Philippe  A. 
Guye,1  who  has  recently  endeavored  to  prove  that  the  amount 
of  rotatory  power  depends  somewhat  on  the  specific  gravity 
of  the  radicals  connected  with  the  asymmetric  carbon.  He 
states  that  if  we  have  two  bodies  CEi  E2  E3  E4,  and  in  one 
the  specific  gravities  are  —  Ej  =  100,  E2  =  101,  E3  =  102, 
and  E4  =  103,  and  in  the  other  Ej  =10,  E2  =  100,  E8  =1000, 

1  Annales  de  Chim.  et  de  Phys.  6,  xxv.  145. 


GENERAL  PRINCIPLES  OF  STEREO-CHEMISTRY.         13 

and  R4  —  10,000,  the  lack  of  symmetry  will  be  much  greater 
in  the  second  case  than  in  the  first,  and  there  should  be  a 
corresponding  difference  in  the  amount  of  optical  activity. 
This  idea  seems  not  to  be  entirely  without  experimental 
verification,  for  M.  Le  Bel  has  found  that  the  angle  for  amyl 
chloride, 

CH3  H 

\     / 

C 
/     \ 

C2H6  CH2C1 

is  1°  6';  for  amyl  bromide  is  4°  24';  and  for  amyl  iodide 
8°  20'.  Moreover,  Guye  has  studied  43  bodies  derived  from 
amyl  chloride  by  replacing  the  CH2C1  by  radicals,  in  which 

.  the  mass  of  the  replacing  radical  is,  as  is  the  case  with 
CH2C1,  greater  than  that  of  the  other  three  groups  com- 
bined with  the  asymmetric  carbon,  and  finds  that  they  are 
all,  like  amyl  chloride  itself,  dextro-rotatory,  his  object 
being  to  show  that  the  direction  as  well  as  the  .amount  of 
optical  activity  depends  upon  specific  gravity.  Now  it  may 
be  that  in  the  simpler  molecules  containing  less  than  three 
carbon  atoms,  there  is  not  sufficient  difference  in  the  specific 
gravities  of  the  radicals  to  cause  appreciable  optical  activity. 
Victor  Meyer's  explanation  of  the  lack 'of  observed  rotatory 
power  in  the  more  simple  molecules  containing  asymmetric 
carbon,  is  that  the  simple  groups  are  more  mobile  than  the 
complex  ones,  and  may  keep  changing  their  positions  in  the 
molecules,  thus  producing  a  tautomerism  "incompatible  with 
the  display  of  optical  activity  >  the  result  being  the  same  as  if 
we  were  dealing  with  mixtures  of  opposite  molecules. 

j      The  number  of  isomeric  forms  possible  for.-any  particular 

I  combination   of   atoms  .depends  upon   the 


14 


THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 


metric  carbon  atoms  which  it  contains.     For  C  (abed),  as  we 
have  seen,  we  may  have  the  two  geometrical  forms,  — 


a     b     c 

\  I  / 

C 

I 

d 


\  I  / 
G 


one  of  which  will  be  dextro  and  the  other  laevo-rotatory.  If, 
however,  in  these  formulas  d  represents  another  asymmetric 
carbon  atom  with  its  attendant  radicals,  two  different  positions 
are  also  possible  for  this  group,  making  four  possible  isomers 
with  two  asymmetric  carbon  atoms.  In  general,  .2^=2",  in 
which  N  =  the  number  of  isomers  and  n  =  the  number  of 
asymmetric  carbon  atoms.  If,  however,  d  =  another  asym- 
metric carbon  exactly  like  the  first,  so  that  we  have  the  sym- 
metrical molecule  C2(abc)2  the  case  is  somewhat  different. 
The  four  possible  configurations  then  become :  — 


Of  these,  (1)  and  (4)  evidently  represent  the  same  body,* 
(4)  being  (1)  inverted,  so  that  in  this  case  the  possible  num- 
ber of  isomers  is  reduced  to  three.  In  general,  if  there  are  n 
asymmetric  carbon  atoms  in  a  symmetrical  formula,  there  are 
theoretically  possible  \  2n  active  isomers  grouped  in  pairs  hav- 
ing equal  and  opposite"  rotatory  power  and  enantiomorphous 
crystalline  form,  and  £  2*  representatives  of  the  inactive 


GENERAL  PRINCIPLES   OF  STEREO-CHEMISTRY.         15 

amesotomic  type.  It  is  true  that  for  most  substances  the 
number  of  isomers  found  does  not  come  up  to  the  number 
predicted  by  this  rule.  For  example,  theory  demands  sixteen 
bodies  of  formula  C6H1208,  whereas  only  fourteen l  have  been 
observed.  In  the  case  of  the  tartaric  acids,  however,  which 
contain  two  asymmetric  carbon  atoms  in  a  symmetrical  for- 
mula, all  of  the  bodies  theoretically  possible  have  been 
obtained. 

It  has  been  noticed  that  an  optically  active  body  sometimes 
loses  its  rotatory  power  simply  upon  heating.  Eacemic  acid, 
for  example,  can  be  made  by  heating  ordinary  dextro-tartaric 
acid.  This  seems  to  be  due  to  the  partial  change  by  heat 
into  the  oppositely  active  body  which  neutralizes  the  rotatory 
power  of  the  body  with  which  we  started,  or  it  may  be  that 
the  mobility  of  the  atoms,  of  which  Victor  Meyer  speaks, 
may  be  increased  by  the  heat,  producing  tautomerism. 

Two  singly  linked  carbon  atoms,  according  to  van't  Hoff, 
should  be  represented  by  two  tetrahedrons  having  the  carbon 
atoms  at  the  same  time  at  the  centre  of  one  and  the  solid 
angle  of  the  other  tetrahedron.  This  would  give  the  figure 
of  two  truncated  pyramids  having  the  plane  of  truncation 
in  common,  but  for  ordinary  purposes  of.  representation  it  is 
generally  considered  sufficient  to  have  the  two  tetrahedrons 
with  a  solid  angle  in  common,  as  has  been  shown  in  the  pre- 
ceding illustrations.  ®> 

It  can  readily  be  seen,  and  is  shown  very  plainly  with 
tetrahedral  models,  that  several  singly  linked  carbon  atoms 
will  lie  in  a  zigzag  line,  or  a  ring,  this  depending  upon  the 
angle  which  the  axes  of  attraction  or  tetrahedral  axes  make 
with  each  other.  This  angle  is  109°  28',  which  is  very 
nearly  the  same  as  the  angle  of  the  regular  pentagon,  108° ; 

i  Am.  Chem.  J.  13,  64. 


16 


THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 


therefore  five  such  atoms  could  readily  form  a  closed  ring, 
and  only  a  slight  deviation  of  the  axes  from  their  normal 
direction  would  be  necessary  in  order  to  produce  a  ring  of  six 
atoms, —  a  consideration  which  may  perhaps  account  for  the 
great  stability  of  five  and  six  atom  carbon  rings. 
The  formula  for  normal  pentane,  as  generally  written, 

(1)  OH. 

(2)  CH, 

(3)  CH, 

(4)  OH, 

(5)  OH. 

gives  an  incorrect  idea  of  the  molecule  in  so  far  as  it  indicates 
that  (1)  must  be  nearer  (3)  than  (4)  or  (5).  Applying  the 
tetrahedral  theory,  the  formulas  for  butane  and  pentane  may 
be  represented  as  follows :  — 


In  "Die  Lagerung  der  Atome  im  Baume,"  van't  Hofi0  starts 
out  with  this  statement:  "The  present  chemical  theory  has 
two  weak  points.  It  takes  into  consideration  neither  the 


GENERAL  PRINCIPLES  OF   STEREO-CHEMISTRY. 


17 


relative  positions  which  the  atoms  assume  in  the  molecule, 
nor  their  kinds  of  motion."  Thus  far  we  have  been  occupied 
principally  with  the  subject  of  the  relative  positions,  and  the 
next  step  in  advance  will  be  to  consider  what  this  theory 
supposes  in  regard  to  the  motions  of  the  atoms.  In  the  first 
place,  it  must  be  conceded  that  any  atomic  motions  in  the 
molecule  must  be  limited  in  such  a  way  that  the  atoms  will, 
in  general,  keep  fixed  relative  positions;  otherwise  isomerism 
would  be  an  impossibility. 

In  considering  the  motion  inside  the  molecule,  van't  Hoff 
starts  with  the  assumption  that  any  such  motion  must  be  reg- 
ular and  periodic  to  account  for  the  constancy  of  properties  of 
the  particular  substance.  In  the  case  of  two  singly  linked 
carbon  atoms,  one  of  the  simplest  conceivable  motions  is  a 
rotation  around  the  axis  joining  the  carbon  atoms.  If  the 
rotation  of  both  atoms  is  in  the  same  direction,  it  becomes 
practically  a  motion  of  the  molecule  as  a  whole,  but  it  is 
assumed  that  the  two  carbon  atoms  may  rotate  in  opposite 
directions  at  the  same  time,  thus  producing  configurations 
such  as  would  result  if  one  of  the  carbon  atoms  was  in  rota- 
tion while  the  other  was  at  rest.  Van't  Hoff's  first  assump- 
tion was  that  such  a  rotation  was  continually  taking  place, 
and  that,  therefore,  the  two  following  formulas, 


or,  more  simply, 


18  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

a     b     e  cab 

\  \  /  \   I  / 

C  C 

I  I 

C  C 

•     /  I  \  /IN 

a     b     c  a     b     c 

(1)  (2) 

should  be  considered  as  representing  the  same  body  in  different 
phases  of  rotation  rather  than  two  different  bodies,  since  (2) 
can  be  derived  from  (1)  by  simple  rotation  of  the  upper  carbon 
atom  to  the  right. 

If  we  deny  the  possibility  of  such  rotation,  and  consider 
each  possible  configuration  of  the  atoms  as  a  distinct  isomeric 
form ,  we  are  led  to  possibilities  of  isomerism  which  are  prac- 
tically infinite.  .  For  example,  in  the  very  simple  formula 
CH3-CH3,  the  three  hydrogen  atoms  of  each  group  might  lie 
in  the  same  vertical  lines  with  the  three  of  the  other  group, 
or  midway  between  these  positions,  or  there  is  a  possibility 
of  any  number  of  arrangements  between  these  two,  so  that  if 
we  consider  each  of  these  possible  arrangements  as  represent- 
ing a  distinct  isomeric  form,  the  number  of  such  possible 
forms  reaches  alarming  proportions.  These  difficulties  vanish, 
however,  if  we  assume  free  rotation,  and  accept  an  hypothesis 
sometimes  referred  to  as  "Van't  Hoff's  second  hypothesis." 
This  is  as  follows :  — 

f  "  When  two  atoms  of  carbon  are  united  by  a  single  bond, 
each  is  capable  of  free  rotation  in  either  direction  about  the" 
common  axis;  and  isomers  maybe  recognized  for  those  bodies 
only  which  cannot  be  brought  into  the  same  configuration  by 
such  rotation." 

In  van't  Hoff's  later  works  he  states,  however,  that  in  all 
probability  there  will  be  one  arrangement  of  the  atoms  in  the 


GENERAL  PRINCIPLES   OF   STEREO-CHEMISTRY.          19 

molecule  which  will  possess  the  greatest  possible  stability, 
and  this  will  be  the  arrangement  which  the  atoms  will  tend 
to  assume  when  the  molecule  is  in  a  state  of  equilibrium.  It 
is  Wislicenus's  idea  that,  if  the  radicals  connected  with  two 
singly  linked  carbon  atoms  are  all  alike,  rotation  will  be 
started  by  the  slightest  impulses  of  heat,  since  there  will  be 
no  one  position  which  will  be  more  stable  than  another,  and 
therefore  nothing  to  determine  a  fixed  position  of  equilib- 
rium. If,  on  the  other  hand,  the  radicals  are  different,  their 
chemical  affinities  will  come  into  play.  The  atoms  or  radi- 
cals not  directly  combined  in  the  molecule  exert  chemical 
attraction  upon  one  another,  the  two  with  greatest  affinity 
tending  to  come  as  near  together  as  possible,  thus  determin- 
ing a  position  of  equilibrium  at  which  the  rotation  will  cease, 
or  tend  to  cease.  It  is  evident  that  more  force  would  be 
required  to  produce  rotation  in  such  a  system,  held  in  a 
definite  position  by  the  mutual  attraction  of  the  parts,  but  it 
is  probable  that  at  sufficiently  high  temperature  there  are 
always  some  configurations  in  a  molecular  aggregate  which 
do  not  correspond  to  the  greatest  attraction.  This  number 
will  increase  with  the  rising  mean  temperature  of  the  mass, 
but  even  at  high  temperatures  the  most  stable  molecules  will 
predominate. 

These  ideas  may  perhaps  be  made  more  plain  by  an  illus- 
tration.    The  three  formulas, 


Br 

H 

COOH 

TT 

COOH 

COOH 

Br 

Br 

H 

\ 

/ 

? 

\ 

/ 

/ 

\ 

/ 

C 

C 

C 

1 

(1) 

(2) 

1      (3) 

c 

c 

C 

/ 

\                             / 

\                     / 

\ 

H 

COOH 

H 

COOH 

H 

COOH 

H 

H 

H 

20  THE    DEVELOPMENT   OF   STEREO-CHEMISTRY. 

according  to  van't  Hoff's  second  hypothesis,  represent  the 
same  molecule  in  different  phases  of  rotation  rather  than 
three  isomeric  modifications,  since  each  can  be  derived  from 
the  preceding  by  simple  rotation  of  the  upper  carbon  atom  to 
the  right;  (3)  would,  however,  probably  represent  the  most 
stable  form,  the  configuration  which  would  be  assumed  in  the 
largest  number  of  molecules,  because  here  the  negative  radi- 
cals COOH  and  Br,  joined  to  one  carbon  atom,  lie  as  near  as 
possible  to  the  hydrogen  of  the  other  carbon  atom;  that  is, 
they  are  in  the  same  vertical  line. 

Recently  there  has  been  some  discussion  as  to  whether  such 
rotation  is  always  possible,  or  whether  it  is  dependent  on  cer- 
tain conditions.  The  idea  of  limited  rotation  was  first  sug- 
gested by  V.  Meyer  in  connection  with  the  oximes,  which  will 
be  discussed  later  when  we  consider  the  stereo-chemistry  of 
nitrogen.  Later  Bethmann l  decided  that,  in  order  to  explain 
the  isomerisin  in  succinic  and  glutaric  acids,  he  must  deny 
free  rotation  in  those  molecules,  and  concluded  that  the  car- 
bon bound  to  carboxyl  cannot  rotate  freely,  but  stops  in  certain 
definite  positions,  and  these  different  positions  correspond  to 
different  isomeric  modifications.  Perhaps  one  of  the  best 
illustrations  of  alleged  limited  rotation  may  be  found  in  the 
benzil  monocarbonic  acids.  Graebe,2  and  Meyer,8  working 
later  with  other  experiments,  proved  that  there  are  two  of 
these  acids  differing  very  slightly  in  chemical  as  well  as 
physical  properties,  and  to  these  two  bodies  they  ascribe  the 
formulas :  — 

1  Zeit.  f.  phys.  Chem.  5,  385.  »  Ber.  23,  2079. 

2  Ber.  21,  2003,  and  Ber.  23, 1344. 


GENERAL  PRINCIPLES   OF   STEREO-CHEMISTRY.         21 


C6H5 


v 


COOH  .  C6H4 


0 


0 


C6H5  0 


and 


\ 


O          C6H4  .  COOH 


In  case  free  rotation  were  possible,  these  formulas,  accord- 
ing to  van't  Hoff,  should  represent  one  and  the  same  body 
instead  of  two  different  ones.  Meyer's  final  conclusion  is 
that  in  the  great  majority  of  cases  we  should  assume  free 
rotation,  for  the  difference  in  character  of  the  constituents 
will  bespeak  a  certain  position  of  equilibrium,  and  rotation 
will  take  place  until  this  position  is  reached;  but  in  those 
cases  in  which  the  radicals  or  atoms  stand  very  near  each 
other  in  electrical  character,  a  state  of  equilibrium  may  be 
reached  in  several  different  ways,  and  so  several  different 
stable  bodies  may  be  formed. 

Eiloart *  has  endeavored  to  show  that  the  rotation  depends 
on  the  temperature  much  as  dissociation  does.  To  do  this 
he  discusses  the  case  of  tolane  tetrachloride,  which  may  be 
represented  by  the  following  formulas :  — 


Cl 
Cl    I    C6H5 

\|X 
C 


Cl        C6H6 

Cl 


Cl 


Cl 


Cl 


C6H5 

Cl 

C6H5 

/ 

\ 

/ 

i 

C 

f      (2) 

OY 

(3) 

r1 

C 

\ 

/ 

\ 

Cl 

C6Hg 

Cl 

Cl 

/°'H' 

1  Guide  to  Stereo-Chemistry,  p.  24. 


Cl 


22  THE    DEVELOPMENT   OF   STEREO-CHEMISTRY. 

These,  according  to  van't  Hoff,  represent  one  and  the  same 
body;  but  (2)  and  (3)  are  the  more  stable  configurations 
which  we  should  expect  the  atoms  to  assume.  If  put  with 
zinc,  we  should  expect  the  two  adjacent  chlorine  atoms  to  be 
extracted;  that  is,  the  two  chlorine  atoms  lying  in  the  same 
vertical  line  in  the  formula,  and  the  remaining  atoms  to 
retain  their  original  relative  positions.  According  to  this, 
(1)  would  yield  the  unstable  dichloride  having  the  formula: 

Cl          C6H6 

\     / 

C 

II 

C 

/    \ 
Cl         C6H5 

and  (2)  and  (3)  would  each  yield  the  stable  dichloride,  having 
the  formula: 

Cl          C6H5 
\   / 
'  C 
II 
C 
/  \ 

C6H5        Cl 

This  experiment  has  been  done  quantitatively,  and  at  80°, 
one-third  of  the  reduction  yield  consists  of  the  unstable 
dichloride  and  two-thirds  of  the  stable  dichloride,  which 
would  seem  to  indicate  that  at  that  temperature  free  rotation 
was  taking  place  and  molecules  of  all  three  kinds  were  pres- 
ent. At  20°,  however,  considerably  more  than  two-thirds  of 
the  reduction  yield  consists  of  the  stable  dichloride,  showing 
that,  at  the  lower  temperature,  the  directive  affinities  come 
into  play  to  stop  the  rotation  at  places  of  stable  equilibrium. 


GENERAL  PRINCIPLES  OF  STEREO-CHEMISTRY.         23 

The  question  then  arises,  can  we  predict  just  what  the 
arrangement  of  the  atoms  inside  the  molecule  will  be  when 
they  have  assumed  the  position  of  stable  equilibrium?  In 
some  cases  it  seems  comparatively  easy.  In  the  tolane  tetra- 
chlorides  given  above,  it  seems  reasonable  to  suppose  that  the 
C6H5  and  Cl  will  tend  to  draw  as  near  together  as  possible, 
and  therefore  that  the  position  of  stability  is  assumed  when 
these  are  in  the  same  vertical  line.  In  many  cases,  however, 
there  is  more  difficulty.  Wislicenus  assumed  that  the  greater 
the  difference  in  positivity  of  two  radicals  not  directly  united, 
the  greater  their  mutual  attraction.  Baeyer  puts  this  in  a 
different  form,  which  in  some  cases  would  coincide  with 
Wislicenus 's  view,  but  in  others  it  would  not.  He  assumes 
that  the  attraction  corresponds  to  what  would  exist  between 
the  two  radicals  if  they  were  directly  connected. 

He  was  led  to  this  difference  from  Wislicenus  by  a  study  of 
the  methyl-succinic  acids,  for  which  Wislicenus  gave  the  fol- 
lowing formulas :  — 

CH8  CH3 

H    I    COOH  H         COOH 

C  C 

I  and  I 

C  C 

\  /i\ 


CH8  CH8        H 


•*-  \_/AJ.g  V-/-1    Aq  JL  J 

COOH  COOH 

(Active.)  (Inactive.) 

But  these  formulas  do  not  show  why  the  inactive  acid 
should  form  its  anhydride  more  readily  than  the  active,  nor 
are  they  in  agreement  with  the  stability  of  the  anhydrides. 
Baeyer,  therefore,  suggests  as  formulas  for  the  two  acids,  — 


THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 


CH5 


\ 


COOH 


H 


CH, 


\ 


COOH 


and 


\ 


COOH 

CH8 

(Active. 


II 


H 


\ 


COOH 


CH3 

(Inactive.) 


assuming  that  methyl  has  a  stronger  attraction  for  methyl 
than  for  carboxyl,  on  the  principle  that  methyl  is  held  more 
strongly  to  methyl  in  ethane  than  to  carboxyl  in  acetic  acid. 
But  to  determine  the  strength  with  which  groups  are  held 
together  in  a  molecule  is  not  always  an  easy  task. 

Bethmann l  has  determined  the  affinity  constants  of  some 
organic  acids  by  measuring  their  electrical  conductivity,  and 
has  proved  that  this  constant  in  a  dibasic  acid  increases  as 
the  distance  between  the  carboxyl  groups  diminishes.  Now 
since  methyl  succinic  acid  has  a  larger  affinity  constant  than 
succinic  acid,  it  may  be  inferred  that  the  introduction  of 
methyl  draws  the  carboxyls  nearer  together.  Dimethyl  suc- 
cinio  acid  has  a  still  larger  affinity-constant,  therefore  the 
addition  of  another  methyl  group  seems  to  draw  the  carboxyls 
still  nearer  together.  Bethmann  explains  these  facts  by  the 
following  formulas :  — 


COOH 


COOH 


H  COOH 


CH3 


Zeitschrift  fur  Phys.  Chemie,  5,  385. 


GENERAL  PRINCIPLES  OF  STEREO-CHEMISTRY.         25 

fr 

In  (1),  the  molecule  is  supposed  to  be  viewed  from  above, 
and  one  tetrahedron  is  turned  half  way  around,  so  that  the 
carboxyl  of  one  tetrahedron  lies  halfway  between  the  two 
hydrogen  atoms  of  the  other.  When  methyl  is  introduced, 
the  attraction  of  methyl  for  carboxyl  draws  the  tetrahedron- 
around  until  the  methyl  and  carboyxl  are  in  "corresponding  " 
positions.  By  the  addition  of  another  methyl,  the  tetrahe- 
drons are  supposed  to  be  drawn  over  towards  each  other  so 
that  the  methyl  and  carboxyl  are  much  nearer  each  other 
than  the  two  hydrogen  atoms. 

Bethmann's  general  hypothesis  is,  however,  not  found  to 
hold  good  in  all  cases,  though  apparently  tenable  for  the  suc- 
cinic  acids  quoted,  since  in  these  acids  the  greater  nearness 
of  the  carboxyls  with  increasing  number  of  methyl  groups  has 
also  been  proved  by  Bischoff 1  in  another  way.  It  is  natu- 
rally assumed  that  the  ease  of  formation  of  anhydrides  will 
increase  as  the  distance  between  the  carboxyls  grows  less. 
Now  Bischoff  has  found  that  while  succinic  acid  is  dehy- 
drated only  with  the  greatest  difficulty ,  the  ease  of  dehydration 
increases  directly  with  the  number  of  methyl  groups,  tetra- 
methyl  succinic  acid  forming  its  anhydride  most  easily.  The 
general  idea  of  the  approaching  of  the  carboxyl  groups  as 
methyl  is  added  is  thus  confirmed,  but  Bischoff' s  formulas 
to  express  and  explain  these  facts  differ  from  those  of  Beth- 
mann.  The  former  holds  that  the  state  of  equilibrium  of  a 
molecule  is  best  expressed  by  assuming  that  the  radicals  are 
as  far  apart  as  possible  instead  of  as  near  together.  This  can 
best  be  represented  on  a  plane  surface,  by  supposing  that  the 
molecule  is  viewed  from  above,  giving  the  figure  of  a  six- 
rayed  star,  as  in  Bethmann's  formula  for  succinic  acid  above. 
Bischoff  farther  holds  that  methyl  and  carboxyl  have  a  repul- 

i  Ber.  23,  620,  and  3419. 


26  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

sion  for  one  another,  and  it  is  this  repulsion  which  drives  the 
carboxyls  nearer  together. 

In  regard  to  the  attractions  and  repulsions  of  methyl  and 
carboxyl,  Bischoff  formulates  the  following  rules :  — 

I.  Carboxyl  is   repelled  by  carboxyl.     In  this   he   agrees 
with  Wislicenus. 

II.  Methyl   is   only   slightly  repelled    by   metnyl.     Von 
Baeyer  states  that  these  radicals  attract,  and  Wislicenus  that 
they  repel  one  another. 

III.  Carboxyl   is   repelled  by  methyl.     This   is  in  direct 
opposition  to  Wislicenus. 

This  diversity  of  opinion  in  regard  to  the  attractions  and 
repulsions  of  the  most  common  organic  radicals  shows  that 
this  branch  of  the  subject  is  still  in  a  very  unsettled  state. 
In  the  simpler  cases,  it  is  possible  to  fix  upon  the  exact 
arrangement  of  the  atoms  in  the  molecule,  when  in  a  state  of 
stable  equilibrium,  with  a  fair  degree  of  probability,  but  in 
many  cases  this  is  impossible)  , 

In  the  case  of  compounds  containing  doubly  linked  carbon 
atoms,  isomerism  has  also  been  observed  which  can  be 
explained  most  readily  on  the  supposition  of  the  carbon  atom 
as  situated  in  the  centre  of  a  tetrahedron\  but  in  this  case  the 
isomerism  does  not  manifest  itself  by  optical^.ctivity,  and  we 
have  no  longer  to  deal  with  the  asymme&ie-eafloon  atom^ 

Two  carbon  atoms,  joined  by  double  bonds,  must,  in  the 
terms  of  our  theory,  be  represented  by  two  tetrahedrons  hav- 
ing two  solid  angles,  or  an  edge,  in  common.  The  four  atoms 
joined  to  the  two  carbons^ j^UjbhenJlie-.iii-a-p'lane.  In  the  for- 
mula C2(R)4  there  will  of  course  be  no  possibility  of  isomerism, 
and  the  same  may  be  said  of  C2(E,i)3K2.  With  02(^1)2(1^2)25 
however,  there  are  three  possible  arrangements :  — 


GENERAL  PRINCIPLES   OF  STEREO-CHEMISTRY.         27 


E 


E 


\ 


\ 


\ 


E2          E: 

(1) 


\ 


\ 


E2 


(2) 


(3) 


(1)  is  immediately  recognized  as  essentially  different  from 
(2)  and  (3),  but  the  tetrahedral  theory  is  called  into  play  to 
account  for  the  differences  between  (2)  and  (3).  As  a  partic- 
ular instance  of  this  kind  we  may  cite  C2H2I2.  Two  bodies 
are  known  having  the  formula  CHI  =  CHI,  differing  in  vola- 
tility, melting  point,  and  specific  gravity.  These  bodies 
could  not  be  explained  upon  any  former  theories,  but  can  be 
readily  explained  on  the  present  hypothesis,  being  repre- 
sented by  the  formulas:  — 


and 


or, 


H          I 

H         I 

\    / 

\  / 

C 

C 

,H 

and                     II 

C 

C 

/    \ 

/   \ 

H         I 

I       H 

Another  illustration  of  this  kind  of  isomerism  is  found  in 
maleic   and  furnaric  acids,  which  have  long  been  known  as 


28  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

bodies  whose   isomerism  was  inexplicable.     This  isomerism 
is  now  explained  by  giving  them  the  formulas, 


H         COOH 

H        COOH 

\   / 

\   / 

C 

C 

II 

II 

C 

C 

/  \ 

/  \ 

H         COOH 

COOH         H 

Maleic  Acid. 

Fumaric  Acid. 

Many  cases  of  this  kind  of  isomerism  are  well  known. 
There  are  many  other  bodies ,  however,  whose  formulas  admit 
of  this  kind  of  isomerism,  in  which  the  phenomenon  has  not 
been  observed.  Butylene,  for  example,  is  known  with  cer- 
tainty only  in  one  form,  although  we  might  expect  two  modi- 
fications represented  by  the  formulas, 

CH3          H 
\   / 

I 

/   \ 
CH8         H  H         CH3 

and  there  are  some  reasons  for  supposing  that  two  such  bodies 
really  exist. 

It  can  readily  be  seen  that  with  C2(Ri)2R2R3>  there  will  be 
three  possible  isomerides  similar  to  those  given  above,  and 
that  with  C2(R1K.2E.8B4)  the  number  of  possible  arrangements 
is  increased  to  six. 

It  must  be  noticed  that  if  we  assume  the  possibility  of  free 
rotation  as  in  the  case  of  singly  linked  carbon  atoms,  then 
the  formulas  for  maleic  and  fumaric  acids  as  given  above 


GENERAL   PRINCIPLES   OF   STEREO-CHEMISTRY.         29 

would  be  identical.  Now  since  isomerides  have  been  ob- 
served, with  slight  differences  in  properties,  which  can  only 
be  explained  by  formulas  similar  to  these,  the  van't  Hoff 
theory  denies  the  possibility  of  free  rotation  in  the  case  of 
doubly  linked  carbon  atoms.  That  free  rotation  is  incom- 
patible with  double  linking  is  well  expressed  to  the  eye  by 
the  tetrahedral  symbols,  ^^we^ve  the  two  tetrahedrons 
with  an  edge  in  common,  it  is  evidentthaT  one^annot  be 
revolved  about  the  axis  joining  the  centres  of  the  two  tetrahe- 
drons without  disturbing  this  arrangement,  or  breaking  the 
bonds  which  hold  together  the  two  atoms >~ 

In  such  bodies,  as  has  been  stated,  there  is  no  optical 
activity,  but  the  isomerides  differ  in  most  physical  proper- 
ties and  also,  to  a  sjj^j^^eii^^J^ejnj^aJ  p^flf T±***}  ^.«; 
for  example,  in  the  ease  of  formation  of  anhydride&though 

V-t_J->*^J^**-N^^^--^  — ^*^    i  -"— *  ~^          .."^^  . 

these  variations  are  not  sufficient  to  demand  a  difference  in 

t~^^^^^  "^^^^^^^••^^^^^^^•^•^^^       "*••--•  ^^-...  - 

atomic  linking.  In  the  case  of  singly  linked  carbon  com- 
pounds just  discussed,  the  differences  in  the  isomerides  are 
limited  almost  entirely  to  optical  properties,  although  the 
divergence  in  other  physical  properties  becomes  somewhat 
greater  as  the  number  of  asymmetric  groups  increases.  \  These 
two  kinds  of  isomerism,  it  will  be  noticed,  are  explained  on 
the  same  general  principle,  but  this  explanation  gives  room 
for  greater  differences  in  properties  in  the  second  case  than 
in  the  first,  since  the  absolute  distance  of  the  atoms  from 
each  other  is  different  in  the  two  isomerides  in  which  the 
carbon  atoms  are  doubly  linked,  whereas,  in  the  singly  linked 
isomerides,  the  individual  atoms  and  radicals  have  exactly 
the  same  position  with  reference  to  each  other  except  for  the 
difference  of  right  and  left,  and  the  distance  apart  of  any  two 
individual  radicals  is  the  same  in  both  cases.  This  answers 
the  main  objection  of  Michael  and  Glaus,  who  have  been  the 

OF  THE 
{  Tf  "WT  T  "XT'  IT  T3  O' 


30          THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 

principal  opponents  of  the  Le  Bel-van 't  Hoff  theory,  and  who 
have  urged  that  the  constitution  of  bodies  which  differ  only 
in  one  single  optical  property  should  not  be  explained  in  the 
same  way  as  the  constitution  of  bodies  which  have  quite  dif- 
ferent chemical  properties,  and  especially  since  the  optical 
activity  is  found  to  depend  upon  external  circumstances,  such 
as  the  solvent  employed,  and  the  concentration  of  the  solution. 

Against  some  particular  cases  explained  in  a  rather  round- 
about way  by  Wislicenus,  Michael  may  bring  well-grounded 
arguments,  but  his  objections  are  not  weighty  enough  to  shat- 
ter the  fundamental  basis  of  the  van't  Hoff-Le  Bel  theory. 
Michael  himself  regards  many  of  these  cases  of  isomerism  as 
inexplicable  on  structural  theories,  but  advances  no  explana- 
tion to  take  the  place  of  the  one  which  he  rejects,  merely 
giving  a  new  name  to  such  bodies  and  calling  them  allo- 
isomeric. 

In  regard  to  triply  linked  carbon  atoms,  there  is  very  little 
to  be  said.  Two  triply  linked  atoms  must  be  represented  by 
two  tetrahedrons  having  three  solid  angles,  or  a  plane,  in 
common.  The  combining  atoms  or  radicals  must,  then,  lie  in 
a  straight  line,  and  there  is  no  chance  for  isomerism  of  any 
kind  nor  for  any  rotatory  motion. 

One  of  the  ideas  which  Wislicenus  first  definitely  formu- 
lated is  that  in  the  formation  of  addition  products  the  atoms 
retain  as  nearly  as  possible  the  relative  positions  which  they 
had  originally;  that  is,  atoms  which,  in  the  formula,  lie  in  the 
same  vertical  line  before  the  change  will  keep  that  relative 
position  after  the  change.  For  example,  taking  a  general 
illustration,  — 


GENERAL  PRINCIPLES   OF   STEREO-CHEMISTRY.         31 


\   / 

C 
II 
C 

/  \ 
b        d 


But  in  changing  from  double  to  single  linkage  these  posi- 
tions will  not  be  permanent  on  account  of  the  rotation  which 
then  becomes  possible.  For  example,  — 


H 

H          H  H 

\   /  \ 

C  C 

II      +  C12  should  give      I 
C  C 


Cl 


\ 


H 


H 


H 


\ 


H 


Cl 


but  now  that  the  carbon  atoms  are  singly  linked,  rotation  of 
one  of  the  carbon  atoms  is  possible,  and  will  proceed  until 
the  state  of  equilibrium  is  attained,  represented  by  the  fol- 
lowing formula,  — 

H 

H    I    Cl 

\l/ 
C 
I 
C 


Cl 


\ 


H 


H 


j  Another  good  illustration  of  this  may  be  found  in  the  maleic 
and  furnaric  acids  previously  mentioned.  They  are  repre- 
sented respectively  by  the  formulas, — 


32 


THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 


H          COOH 
\    / 

C 

II      (1)         and 
C 

/  \ 


H        COOH 
\  / 

C 

II      (2) 

C 
/  \ 


H        COOH  COOH        H 

If  we  add  H  Br  to  (1)  we  should  get ,  — 


COOH 


COOH 


H 


\ 


H 


H 


\ 


H 


f1 


I      (3)      and  to  (2)         I     (4) 
C  C 

\  /\ 


H 


Br 


COOH 


COOH         Br 
H 


By  rotation  of  the  lower  carbon  atom  in  (3)  we  get  an 
arrangement  of  atoms  similar  to  that  in  (4),  which  is  pre- 
sumably a  more  stable  form,  and  here  we  have  an  explanation 
of  the  well-known  fact  that  both  maleic  and  fumaric  acids  are 
converted  by  hydrobromic  acid  into  monobrom-succinic  acid. 

In  snch  cases  as  the  latter,  where  there  are  two  different 
radicals  attached  to  one  of  the  carbon  atoms,  Wislicenus 
points  out  the  possibility  of  obtaining  two  geometrical  isome- 
rides.  Thus,  in  general  form,  — 


C 


\ 


d 

a    I     b 
\l/ 

C 
I 
C 

X|\ 

a         a 

d 


d 


\ 


or 


\ 


GENERAL  PRINCIPLES   OF   STEREO-CHEMISTRY.         33 

which  are  geometrical  isomers.  It  would  naturally  be 
expected  that  both  of  these  bodies  should  be  formed  at  the 
same  time  in  about  equal  quantities,  and  this  explains  why, 
in  the  formation  of  such  addition  products,  one  never  obtains 
substances  which  rotate  the  plane  of  polarized  light. 

The  above  formulas  also  show  that  optically  opposite  modi- 
fications produce  identical  products  if  they  go  over  into  unsat- 
urated  compounds  through  the  loss  of  the  same  radical.  For 
example,  if  d2  be  withdrawn  from  the  geometrical  isomers 
given  above,  the  result  will  be  one  and  the  same  body,  — 

a        b 

\   / 

C 

II 

C 

/  \ 
a         a 

Wislicenus  also  uses  his  principle  of  least  disturbance  in 
the  formation  of  addition  products,  to  determine  the  configu- 
ration of  particular  molecules.  One  or  two  simple  illustra- 
tions of  this  may  suffice  at  this  point.  Two  tolane  dichlorides 
are  known.  The  one  with  the  higher  melting-point  is 
obtained  by  the  direct  addition  of  chlorine  to  tolane,  and  must, 
therefore,  be  plane-symmetrical,  having  the  formula, 

C6H5        Cl 
\  / 

C 
II 
C 

/  \ 
C6H5        Cl 

leaving  the  centre-symmetrical  formula, 

3 


34  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

C6H5          01 
\    / 

c 

It 

c 

/  \ 

Cl        C6H6 

for  the  dichloride  with  lower  melting-point. 

It  would,  in  general,  be  expected  that  the  body  with  centre- 
symmetrical  formula  should  be  more  stable  than  its  plane- 
symmetrical  isomer,  and  this  is  found  to  be  true  in  the  cases 
where  the  configuration  can  be  determined.  Now  two  differ- 
ent iodo-ethylenes  are  found  to  result  from  adding  iodine  to 
acetylene.  One  is  liquid  and  unstable,  and  the  other  soli<ji 
and  stable.  Wislicenus  concludes  that  the  liquid  body  must 
be  plane-symmetrical, 

H        I 
\  / 

C 
H 
C 

/  \ 
H        I 

and  must,  therefore,  be  the  one  formed  by  the  direct  action 
of  iodine  on  acetylene,  while  the  other  modification  must  be 
produced  by  a  secondary  reaction.  To  explain  the  simul- 
taneous formation  of  this  second  modification,  he  assumes 
that  a  tetra-iodide  is  first  formed, 

I 

H    I    I 

\l/ 
C 

I 

C 

/l\ 

I  I  I 

H 


GENERAL  PRINCIPLES  OF  STEREO-CHEMISTRY.         35 

and  that  T2  is  given  off  to  more  acetylene,  leaving  the  solid, 
stable,  centre-symmetrical  iodide, 

I        H 
\   / 

C 
II 
C 

/  \ 
H        I 

Briefly  summarized,  we  have  shown  in  the  foregoing  that 
isomerism  is  of  three  kinds:  — 

1.  Ordinary  chemical  isomerism,  explained  by  difference  of 
a1|)mic  linking. 

2.  Physical   or    optical    isomerism.      Differences   limited 
almost  entirely  to  difference  in  optical  properties  and  crystal- 
line form.     The  optical  activity  is  due  to  the  fact  that  the 
molecule  possesses  no  plane  of  symmetry.     The  existence  of 
an   optically  active   body  implies   the   existence   of  another 
equally  but   oppositely  active,  and  the  configuration  of  the 
molecules  of  these  two  bodies  is  such  that  they  bear  to  each 
other  the  relation  of  an  object  and  its  image  reflected  in  a 
mirror.     Considering  the  four  valencies  of  the  carbon  atom  as 
extended  in  the  directions  of  the  four  angles  of  a  tetrahedron, 
the  two  optically  active  bodies  may  be  represented  by  right 
and  left  tetrahedra. 

3.  Stereo-chemical  isomerism.      The  amount  of  difference 
in  properties  lies  midway  between  (1)  and  (2).     There  are 
differences  in  physical  properties,  and  also  to  a  slight  extent 
in  chemical  properties,  but  not  sufficient  to  indicate  a  differ- 
ence  in   atomic.-  linking.      Found  principally  in   connection 
with  compounds  containing  two  doubly  linked  carbon  atoms. 
Explained  on  supposition  of  different  arrangement  in  space  of 
the  radicals  combined  with  the  carbon  atoms. 


36     THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

The  principal  suppositions  concerning  the  carbon  atom, 
which  underlie  the  generally  accepted  stereo-chemical  theories 
are  as  follows :  — 

1.  The  carbon  atom  possesses  four  equal  valencies. 

2.  Observed  facts  are  best  explained  by  representing  the 
carbon  atom  as  situated  in  the  centre  of  a  tetrahedron  with 
its  four   valencies  extending   in  the  directions  of  the   four 
solid  angles. 

3.  Two  singly  linked  carbon  atoms  are  represented  as  two 
tetrahedra   having   a   solid   angle   in   common;    two   doubly 
linked  carbon  atoms  as  two  tetrahedra  with  an  edge  in  com- 
mon; and  two  triply  linked  carbon  atoms  as  two  tetrahedra 
with  a  face  in  common. 

4.  The  motion  of  the  atoms  inside  the  molecule  must  be 
limited  in  extent,  to  account  for  isomerism ;  and  of  a  periodic 
nature  to  account  for  constancy  of  properties.v- 

5.  In  the  case  of  two  singly  linked  carbon  atoms,  the  rota- 
tion of  one  atom  with  its  attendant  radicals  around  the  axis 
joining  the  two  atoms  is  considered  possible,  but  except  at 
high  temperatures,  this  rotation  ceases  at  some  point  deter- 
mined by  the  chemical  affinities  of  the  radicals  which  are  not 
directly  connected. 

6.  In  the  case  of  doubly  and  triply  linked  carbon  atoms, 
no  rotation  around  an  axis  is  possible. 


II. 


ILLUSTRATIONS   AND  APPLICATIONS   TO 
PARTICULAR  CASES. 

SINCE  the  value  of  any  theory  depends  upon  the  number  of 
facts  with  which  it  is  found  to  be  in  harmony,  it  may  be 
well,  before  taking  up  the  more  specialized  part  of  this  work, 
to  consider  the  application  of  the  general  stereo-chemical 
theories  to  some  well-known  organic  bodies.  Such  examples, 
it  is  to  be  hoped,  will  be  of  value  in  giving  a  clearer  idea  of 
the  theories,  and  may  at  the  same  time  enable  us  to  see  in 
how  far  these  theories  are  in  accordance  with  observed  facts. 

Among  the  more  common  substances  in  which  optical  activ- 
ity has  long  been  noticed  may  be  mentioned  the  following 
substances,  the  formulas  for  which  show  that  each  contains 
an  asymmetric  carbon  atom :  — 
amyl  alcohol, 

H          CH3 
N  / 

C 
/   \ 

C2H5          CH2OH ; 

ethylidene  lactic  acid, 

COOH         CH3 
\  / 

C 

/  \ 
H          OH; 


38  THE  DEVELOPMENT  OF  STEKEO-CHEMISTRY. 

malic  acid, 

COOH         CH2COOH 
\  / 

c 
/  \ 

H          OH; 


amylethane, 


aspartic  acid, 


and  asparagine, 


CH8         C2H5 
\  / 

C 

/  \ 
H          C8H7; 

H          NH2 
\   / 

C 

/  \ 

COOH         CH2 
I 
COOH 

H         NH2 
\  / 

C 

/  \ 
COOH         CH2  • 


Two  ethylidene  lactic  acids  have  long  been  known.  Of 
these  the  fermentation  lactic  acid  is  inactive,  and  sarco-lactic 
acid  is  dextro-rotatory.  In  December,  1890, l  by  the  action 
of  a  micro-organism  on  cane-sugar,  an  ethylidene  lactic  acid 
was  obtained  which  was  laevo-rotatory.  We  have,  then,  the 
two  optically  active  bodies  demanded  by  theory,  and  it  only 
remained  to  prove  that  the  fermentation  or  inactive  acid  is 
made  up  of  these  two  active  isomers,  —  a  work  which  has 

1  Am.  Chem.  Journ.  xiii.  277. 


ILLUSTRATIONS  AND  APPLICATIONS.  39 

recently  been  accomplished.  Inactive  lactic  has  now  been 
separated  into  its  optically  active  components  by  three 
methods.  Frankland  and  MacGregor 1  obtained  sarco-lactic 
acid  by  the  fermentation  of  inactive  lactic  acid,  but  found 
that  if  the  fermentation  was  interrupted  too  early,  the  active 
was  mixed  with  a  large  quantity  of  inactive  acid,  and  if  con- 
tinued too  long,  the  active  lactate  was  also  destroyed.  Active 
components  were  also  obtained  from  the  inactive  acid  by  crys- 
tallization of  salts  of  the  alkaloids,  and  finally  Purdie 2  has 
found  that  the  method  of  spontaneous  resolution  by  crystal- 
lization can  be  applied  with  success. 

The  case  of  malic  acid  is  very  similar  to  that  of  lactic  acid. 
Three  different  bodies  are  known,  having  the  same  structural 
formula;  one  is  inactive,  one  is  dextro-rotatory,  and  the  other 
laevo-rotatory ;  and  the  inactive  malic  acid  has  been  proved  to 
be  identical  with  that  obtained  by  mixing  equal  quantities  of 
the  dextro  and  laevo  acids. 

Active  malic  acid  gives  by  reduction  succinic  acid,  which 
is  inactive.  The  reaction  is  expressed  by  the  following 
equation :  — 

H          OH  ff         H 

\  /  \  / 

G  +  H2  =  H20  4-  <JI 

/  \  /  \ 

COOH         CH2CGOH  COOH         CH2COOH 

By  this  equation,  the  carbon  atom  is  shown  to  have  lost  its 
asymmetry  at  the  same  time  that  the  body  loses  its  activity. 
On  the  other  hand,  right  malic  acid  is  produced  by  the  reduc- 
tion of  right  tartaric  acid,  in  this  case  the  resulting  body  as 
well  as  the  original  one  containing  asymmetric  carbon. 

1  J.  Chem.  Soc.  63,  1028,  2  Trans.  Chem.  Soc.  63,  1143. 


40  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

The  sugars  have  already  been  mentioned  as  optically  active 
bodies  containing  asymmetric  carbon,  and  with  them  belong 
the  mannite  and  saccharic  acid  groups.  In  these  latter 
bodies,  there  are  four  asymmetric  carbon  atoms  and  a  sym- 
metrical arrangement,  so  that  theory  predicts  that,  for  each 
formula,  there  should  be  eight  active  and  two  inactive 
bodies.  Of  the  ten  theoretically  possible  bodies  of  formula 
C4H4(OH)4(COOH)2,  eight  have  already  been  obtained. 

Such  illustrations  as  these  might  be  almost  indefinitely 
extended,  but  it  should  be  noticed  again  that  the  mere  fact 
that  the  resulting  body  contains  asymmetric  carbon  does  not 
necessarily  imply  that  it  will  be  active.  For  example, 
Kekule  obtained  by  action  of  hydrobromic  acid  on  active 
malic  acid,  monobromsuccinic  acid,  which  is  inactive,  though 
its^  formula 

H          Br 
\    / 

C 

/   \ 
COOH         CH2COOH 

shows  that  it  contains  an  asymmetric  carbon  atom. 

It  should  perhaps  be  mentioned  here  that  optical  activity 
has  been  rarely  observed  in  halogen  derivatives.  This  has 
been  so  noticeable  that  some  investigators  have  concluded 
that  the  presence  of  a  halogen  element  is  incompatible  with 
optical  activity.  This,  however,  has  been  proved  not  to  be 
absolutely  the  case,  since  Le  Bel  has  discovered  rotatory 
power  in  amyl  iodide,  and  Guye  in  the  corresponding  bromide 
and  chloride. 

Phenyl-brom-lactic  acid l  has  also  been  recently  converted 
by  cinchonine  into  a  right  and  left  handed  modification,  but 

i  Lieb.  Ann.  d.  Chera.  271,  159. 


ILLUSTRATIONS  AND  APPLICATIONS.  41 

in  this  case  the  optical  activity  may  be  due  to  the  asymmetry 
of  the  carbon  atom  not  directly  connected  with  bromine. 
Erlenmeyer  has  also  transformed  this  latter  body  into  active 
phenyl-oxacrylic  acid,  which  is  interesting  from  the  fact  that 
in  this  case  the  two  asymmetric  carbons  are,  according  to 
Glaser's  formula,  in  a  ring, 

O 

/  \ 
C6H5  -  C C  -  COOH 

H         H 

L.  Meyer,  jun.,1  and   C.  Liebermann2  have  succeeded  in 
decomposing  cinnamic  acid  dibromide 

C6H5  -  CHBr  -  CHBr  -  COOH, 


by  strychnine  "into  two  active  isomers ;  and  in  this  case  it  is 
evident  that  the  activity  must  be  due  to  the  asymmetry 
of  a  carbon  atom  directly  united  with  a  halogen  element. 
Recently,  also,  Walden  8  has  shown  that  active  chlorsuccinic 
acid  can  be  produced  from  active  malic  acid;  so  that  although 
many  efforts  to  prepare  active  asymmetric  halogen  compounds 
have  met  with  failure,  enough  such  compounds  have  been 
produced  to  show  that  the  presence  of  a  halogen  is  not  abso- 
lutely fatal  to  optical  activity. 

Certain  reactions  have  been  observed  which  result  in  the 
splitting  off  from  a  molecule  of  atoms  apparently  quite  re- 
mote from  each  other.  For  example,  lactones  are  formed  by 
the  splitting  off  of  water  from  a  carboxyl  and  hydroxyl  group 
which  are  in  the  y  or  8  position  relatively  to  each  other 
rather  than  in  the  a  position  as  might  be  expected  from  the 

i  Ber.  25,  3121.  2  Ber.  26,  245.  *  Ber.  26,  210. 


42 


THE  DEVELOPMENT   OF    STEREO-CHEMISTRY. 


ordinary  plane  formulas.  The  use  of  the  tetrahedral  figures, 
however,  as  given  below,  explains  this  readily,  showing  that 
the  hydroxyl  and  carboxyl  groups  may  be  much  nearer  each 
other  when  in  the  y  or  8  position  than  they  could  be  in  the  a 
or  ft. 


Another  illustration  of  this  same  kind  may  be  found  in  the 
fact  that  the  a  and  ft  amido  acids  do  not  lose  water  spontane- 
ously, but  the  y  and  8  acids  do  so  easily.  The  same  explana- 
tion may  also  be  applied  to  the  fact  that  the  reaction  expressed 
by  the  following  equation 


XCOOH 
\COOH 


=  H20  +  CnH2 


. 


does  not  take  place  with  oxalic  or  malonic  acids,  but  only 
with  succinic  and  glutaric  acids  j  that  is,  in  those  acids  in 
which  the  two  carboxyl  groups  are  in  the  y  and  8  positions 
relatively  to  each  other. 

This  idea  of  the  singly  linked  carbon  atoms  being  arranged 
in  a  ring  rather  than  in  a  straight  line,  is  also  in  best  agree- 
ment with  the  fact  that  amyl  nitrate  is  converted  by  P206  into 
pyridin,  as  is  shown  by  the  following  equation,  — 


ILLUSTRATIONS  AND  APPLICATIONS. 


43 


HH 
C 

/  \ 
HHC        CHH 

I          I        -3H20 
CHH 


HC 
HH 


H 
C 

/  ^ 

HC        CH 

II          I 

N        CH 

\  <? 

C 

H 


In  compounds  where  NaBr  and  C02  split  off  from  the  same 
molecule,  it  has  been  found  that  the  sodium  and  bromine 
occupy  the  ft  position  relatively  to  each  other  rather  than 
the  a.  A  geometrically  expressed  equation  shows  the  cause 
of  this :  — 


=    NaBr    + 


ONa 


On  account  of  the  affinity  between  the  sodium  and  the  bro- 
mine, they  tend  to  bend  the  tetrahedrons  from  their  original 
position  until  they  get  near  enough  so  that  these  atoms  split  off. 

When  we  attempt  to  apply  our  theories  to  doubly  linked 


44          THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

carbon  compounds,  the  case  becomes  rather  more  compli- 
cated, because  we  have  here  bodies  differing  from  each  other 
in  a  number  of  physical  properties,  such  as  boiling  and  melt- 
ing points,  crystalline  form,  etc.  ,  and  also  to  a  slight  extent 
in  chemical  properties,  so  that  in  many  cases  the  question 
arises  as  to  whether  such  bodies  should  be  considered  as  geo- 
metrical isomers  or  structurally  different  bodies. 

Among  the  most  common  of  such  cases  of  isomerism,  which 
have  aroused  much  discussion  as  to  the  cause,  is  that  of 
maleic  and  furnaric  acids.1  It  was  at  one  time  suggested  that 
one  of  these  acids  was  a  polymeric  form  of  'the  other,  but  on 
experiment  their  vapor  densities  were  found  to  be  identical, 
which  gave  the  death-blow  to  that  theory. 

The  Kolbe-Fittig  explanation  was  that  in  maleic  acid  there 
was  a  divalent  carbon  atom,  according  to  the  formula, 

COOH  -  CH2  -  C  -  COOH. 

Such  an  explanation  as  this  would  seem  to  demand  the  pos- 
sibility of  a  similar  isomerism  in  ethylene  itself,  and  all  of 
its  derivatives,  —  an  isomerism  which  it  is  needless  to  say  has 
not  yet  been  observed. 

Another  explanation  is  given  by  Anschutz,who  has  assigned 
to  maleic  acid  the  formula, 


/  IXOH 

/  CH 

0  II 

\  CH 

\C  =  0 

i  Ann.  Chem.  Pharm.  246,  53.  Bull.  Soc.  Chim.  (2)  37,  300.  Am.  Chem. 
Journ.  9,  253  &  364.  Michael,  —  Untersuchungen  ueber  Alloisomerie.  Wisli- 
cenus,  —  Ueber  die  Raumlichen  Anordnung  der  Atome  in  Organischen 
Molekiilen,  &c. 


ILLUSTRATIONS  AND  APPLICATIONS. 


45 


giving  only  to  fumaric  acid  the  formula 

COOH 
I 

CH 
II 

CH 
I 
COOH 

These  formulas  are  in  agreement  with  facts  in  so  far  as  they 
would  indicate  that  the  maleic  acid  was  less  stable  than  its 
isomer,  and  readily  formed  its  anhydride;  but  this  explana- 
tion could  only  be  applied  to  acids  of  this  type,  whereas  a 
similar  isomerism  has  been  observed  in  a  number  of  other 
cases,  and  there  should  therefore  be  a  more  general  expla- 
nation. 


The  general  consensus  of  opinion 
acids  the^same  structural  formula, 


to  these  two 


According  to  the  van't  Hoff  hypothesis,  the  atoms  situated 
in  the  angles  of  two  tetrahedrons,  having  two  solid  angles  in 
common,  admit*T5i^two"aiiaiigUlht]uLy,  day 


H         COOH 


H 


COOH 


\ 


and 


H 


\ 
COOH 

(1) 


C 
II 
C 

/  \ 
COOH         H 

(2) 


Of  these  (1)  has  been  assumed  to  be  the  arrangement  in 
maleic  acid,  as  indicating  the  less  favorable  configuration  for 


46  THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

stability,  —  and  inaleic  acid  is  well  known  to  be  less  stable 
than  fumaric,  —  and  also  explaining  the  fact  that  maleic 
anEyctri'de  can  be  readily  formed,  whereas  a  similar  reaction  is 
impossible  in  the  case  of  fumaric  acid. 

Th"5re  arecertam  reactions  of  these  bodies  which  seem 
especially  well  adapted  to  illustrate  the  theories  given  in  the 
preceding  chapter,  and  they  will  therefore  be  briefly  noticed 
here. 

Fumaric  acid  may  be  made  by  treating  bromsuccinic  acid 
with  alcoholic  potash.  The  most  favorable  configuration  for 
bromsuccinic  acid  is,  — 

H 
H    I    COOH 


C 

/l\ 

COOH    I    H 
Br 

Upon  treatment  with  potassium  hydroxide  the  hydrobromic 
acid  is  withdrawn,  and  there  is  left  the  arrangement  as 
assigned  above  to  fumaric  acid. 

Both   acids  are   converted   into   succinic   acid  by  nascent 
hydrogen, 

H 

COOH         H  COOH    I    H 

\  /  \|X 

C  C 

II  +  H2  =  I         (1) 

C  C 


/  \ 

H         COOH  H 


\ 


COOH 
H 

and 


ILLUSTRATIONS  AND  APPLICATIONS.  47 

H 

COOH         H  COOH        H 

\  X  \  X 

C  C 

II  +  H2  =  I         (2) 

C  C 

X    \  X|\ 

COOH         H  COOH    I    H 

H 

but  by  rotation  of  the  lower  carbon  atom  in  (2)  we  should  get 
the  same  arrangement  as  in  (1),  which  is  the  most  favorable  A 
configuration  for  stability  for   succinic   acid.     Similarly,    it 
may  be   shown  that   both   bodies  would   be   converted   into 
bromsuccinic  acid  by  action  of  hydrobromic  acid. 

Again,  if  hydrobromic  acid  be  removed  from  the  bromine 
addition  product  of  maleic  acid,  the  result  is  bromofumaric 
acid,  and  if  fumaric  acid  be  subjected  to  similar  treatment, 
the  result  is  bromomaleic  acid.  These  facts  are  represented 
by  the  following  equations :  — 

Br  COOH 

COOH         H                COOH    I    H  H         Br 

\  X                                 \|X  \  X 

C                                   C  C 
||            +  Br2  =              I        =  (by  rotation)          I 

C                                     C  C 

X    \                                      X|\  X   \ 

COOH         H                COOH    I    H  COOH        H 

Br  Br 

Then,  by  withdrawing  the  HBr/we  obtain:  — 

H         COOH 
\   X 

C 
II 
C 


UNIVERSITY) 


48  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

The  other  reaction  may  be  similarly  explained  as  shown  by 
the  following  formulas,  — 


H         COOH  H 

\  /  \ 


Br  H 


COOH  COOH 


\ 


Br 


C  C  C 

||  +  Br2  =  |        =  (by  rotation)          | 

C  C  C 

/  \  /|\  /|\ 

COOH         H  COOH    I    H  COOH    I    H 

Br  Br 

Then,  by  withdrawal  of  HBr  = 

COOH         H 
\  / 

C 
II 
C 

/  \ 
COOH         Br 

When  malic  acid  is  heated  to  150°,  both  fumaric  and 
maleic  acids  are  formed,  but  the  former  is  much  more  abun- 
dant than  the  latter.  The  formula  for  malic  acid  is :  — 


COOH  H 


H 


\ 


OH  OH 


\ 


COOH 


H 


C  C 

I      (1)  or,  by  rotation,        |      (2) 
C  C 

H  H         H 


COOH  COOH 

On  subtracting  H20  from  (1)  we  get  readily :  — 


ILLUSTRATIONS  AND  APPLICATIONS.  49 

H  COOH 

\  / 

c 

II 
c 

/  \ 

H         COOH 

and  from  (2) :  — 

H         COOH 

\  / 

C 

II 

C 

/  \ 
COOH      .   H 

but  since  (2)  is  the  configuration  more  favorable  to  stability, 
we  should  expect  more  molecules  of  the  malic  acid  to  possess 
this  arrangement  than  the  other,  and  therefore  to  have  more 
fumaric  than  maleic  acid  produced.  According  to  the  theories 
enunciated,  we  should  also  expect  that  more  of  the  less  stable 
molecules  of  malic  acid  could  exist  at  a  higher  temperature, 
and  therefore  that  the  decomposition  yield  of  maleic  acid 
would  increase  with  rise  in  temperature.  This  has  been 
found  to  be  the  case.  Michael  argues  that  no  conclusions 
should  be  drawn  from  this  reaction,  because  by  further  action 
of  heat  and  water  some  of  the  maleic  acid  first  formed  must 
be  converted  into  fumaric  acid.  To  obviate  this  source  of 
error,  he  did  a  similar  experiment  in  vacua  at  a  temperature 
at  which  the  maleic  acid  and  water  would  be  volatilized,  and 
thus,  as  far  as  possible,  removed  from  further  action.  Done 
in  this  way  he  found  that  at  180°-190°,  he  obtained  from 
malic  acid,  to  every  100  parts  of  fumaric  acid,  27  parts  of 
maleic  acid,  and  at  200° -205°,  52  parts  of  maleic  acid  to 
every  100  of  fumaric  acid.  Under  the  conditions  under  which 

4 


50  THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

Wislicenus  did  his  experiments  he  found  at  180°-190°, 
9-10  parts  of  maleic  acid  to  100  of  fumaric,  and  at 
200°-205°  about  12  parts  of  maleic  to  100  of  fumaric.  This 
shows  that  the  relative  proportions  which  can  be  obtained 
depend  largely  on  the  conditions  under  which  the  experiment 
is  performed,  but  the  general  results  are  similar  in  the  two 
cases ;  the  amount  of  fumaric  acid  formed  seems  to  be  greater 
than  that  of  the  maleic,  and  the  proportion  of  the  maleic 
increases  with  the  temperature. 

Although  many  of  the  reactions  of  these  acids  are  in  com- 
plete accordance  with  the  theories  given  and  are  readily 
explained  by  them,  there  are  others,  besides  the  one  just  men- 
tioned, upon  which  some  douBt  has  been  cast,  which  cannot 
be  so  easily  explained  by  these  theories.  One  of  these  is  the 
change  of  maleic  into  fumaric  by  [the  action  of  strong  acids, 
especially  HBr.  Wislicenus  attempts  to  vindicate  his  theory 
by  supposing  that  bromsuccinic  acid  is  formed  as  an  inter- 
mediate product  according  to  the  following  formulas :  — 


COOH         H 

\  / 

C 

II 

C 

/  \ 
COOH         H 


Then  the  HBr  splits  off  again,  leaving  — 

H         COOH 
\  / 
C 


C 

/  \ 
COOH         H 


H 

H 

COOH 

H 
> 

H 

COOH 

G 

C 

= 

= 

(by  rotation) 

1 

C 

C 

/ 

\ 

/  \ 

COOH 

H 

COOH 

H 

Br 

Br 

ILLUSTRATIONS  AND  APPLICATIONS.  51 

The  objection  urged  against  this  explanation  is  that  the 
bromsuccinic  acid  is  stable  and  does  not  yield  fumaric  acid 
in  presence  of  fuming  hydrobromic  acid.  This  objection  is  a 
strong  one,  but  does  not  seem  to  be  absolutely  insuperable. 
It  is  conceivable  that  reactions  may  be  possible  in  a  molecule 
just  formed  in  a  changing  system,  which  would  not  be  pos- 
sible in  a  mass  of  these  molecules,  after  their  stability  had 
become  fixed.  However,  Skraup  l  has  made  some  investiga- 
tion of  this  change,  and  also  concludes  from  his  experiments 
that  Wislicenus's  explanation  is  insufficient.  He  finds  that 
the  change  from  maleic  to  fumaric  acid  takes  place  through 
the  influence  of  acid  or  water,  and  is  also  possible  through 
other  chemical  processes.  Only  those  acids  which  act  chem- 
ically on  maleic  acid-  produce  the  change  easily,  though  acids 
whose  constituents  cannot  be  added  to  the  maleic  acid,  as 
nitric  and  sulphuric  acids,  can  also  produce  this  transforma- 
tion. It  might  be  urged  that  by  the  addition  of  water  malic 
acid  is  formed  as  an  intermediate  product;  but  the  fact  that 
malic  acid  itself  is  not  changed  to  fumaric  under  the  condi- 
tions of  these  experiments  makes  this  explanation  improbable. 

The  explanation  of  the  change  which  is  adopted  by  Skraup, 
and  also  by  Bischoff,  is  that  small  quantities  of  an  addition 
product  are  formed,  that  this  is  not  an  intermediate  product, 
but  that  by  its  catalytic  action  the  transformation  of  maleic 
to  fumaric  acid  takes  place.  As  Bischoff  expresses  it,  the 
addition  is  the  causal,  and  the  transformation  the  catalyti- 
cally  impelled  process.  As  a  general  explanation  of  this 
and  all  catalytic  action,  Skraup  states  that  in  many  chemical 
processes  oscillations  result,  which  may  produce  in  other 
chemically  disinterested  molecules  other  oscillations,  which, 
either  by  themselves  or  supported  by  other  momenta  such  as 

1  Monatshefte  f.  Chemie,  12,  107. 


52  THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

heat  motions,  may  effect  a  total  change  in  the  structure  of 
the  system. 

Another  reaction  which  is  more  difficult  to  dispose  of  than 
the  preceding  in  that  it  seems  to  really  contradict  a  portion 
of  the  generally  accepted  theories,  is  that  which  takes  place 
when  bromine  is  added  to  acetylenedicarboxylic  acid.  Ac- 
cording to  Wislicenus,  this  reaction  should  give  dibrommaleic 
acid :  — 

COOH  Br         COOH 

I  \  / 

C  C 

III  +  Br2  =  II 

C  C 

I  /  \ 

COOH  Br         COOH 

Bandrowski  obtained  as  the  result  of  this  reaction  prin- 
cipally dibromfumaric  acid.  Michael  has  repeated  the  experi- 
ments and  finds  both  acids,  about  30%  being  dibrommaleic, 
and  most  of  the  remaining  70%  being  dibromfumaric.  Wis- 
licenus reports  as  the  result  of  his  own  experiments  that 
dibrommaleic  acid  is  the  first  product  of  the  reaction,  but 
that  hydrobromic  acid  is  formed  at  the  same  time,  and  if 
great  care  is  not  taken,  a  secondary  reaction  will  take  place 
and  dibromfumaric  acid  be  formed  by  the  action  of  hydro- 
bromic acid  upon  the  dibrommaleic  acid.  He  gives  great  prom- 
inence to  the  part  played  by  HBr  in  this  reaction.  Meanwhile 
Michael  states  that  he  has  left  brommaleic  acid  mixed  with 
hydrobromic  acid  under  the  same  conditions  as  those  in  which 
the  reaction  with  the  acetylenedicarbonic  acid  took  place,  and 
did  not  find  that  bromfumaric  acid  was  formed.  Finally, 
having  proved  to  his  own  satisfaction  that  both  acids  are 
normal  addition  products  in  the  presence  of  water,  he  repeated 
experiments  using  an  ether  of  the  acetylenedicarbonic  acid, 


ILLUSTRATIONS   AND  APPLICATIONS.  53 

and  bromine  dissolved  in  carbon  tetrachloride,  to  prevent  the 
formation  of  hydrobromic  acid,  and  obtained  the  same  results. 
For  an  outsider  who  does  not  see  these  experiments  per- 
formed, it  is  difficult  to  decide  whose  results  to  accept  as 
trustworthy,  but  it  is  safe  at  least  to  say  this,  that  we  have 
here  a  reaction  which  has  not  been  satisfactorily  explained  in 
terms  of  the  theories  given,  and  which  casts  some  doubt  upon 
the  hypothesis  suggested  by  Wislicenus;  namely,  that  in 
changing  from  one  form  of  linkage  to  another,  the  atoms 
keep  their  original  relative  positions. 

The  isomerism  in  the  acrylic  acid  series  1  has  also  been  the 
cause  of  much  study  and  discussion.  The  fi£SjLji]£mb6.r  of 
the  series,  acrylic  acid,  has  the  formula, 

H         COOH 

\  / 

C 

II 

C 

/  \ 
H         H 

which  shows  no  possibilities  of  isomerism.  In  accordance 
with  this,  only  one  acrylic  acid  has  ever  been  obtained.  For 
the  second  member  of  the  series,  C3H5(COOH),  several  dif- 
ferent arrangements  are  possible.  From  a  common  method  of 
preparation  of  the  a  or  solid  crotonic  acid,  by  treatment  of 
allyl  cyanide  with  potassium  hydroxide,  it  was  naturally 
given  the  formula :  — 

1  Ber.  20,  1008,  1010;  Ann.  Chem.  Pharm.  248,  281 ;  Journ.  prak-Chem. 
(2)  35,257;  Journ.  prak-Chem.  (2)  38,6;  Journ.  prak-Chem.  (2)  36,  174; 
Journ.  prak-Chem.  (2)  38, 1 ;  Michael's  Untersuchungen  ueber  Alloisomerie. 


54  THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

H  H 

\    / 

C 
II 
C 

/  \ 
H         CH2 .  COOH 

but  Kekule  showed  that  its  behavior  toward  fused  alkalies 
was  not  in  accordance  with  this  formula,  and  that  it  should 
be  written  CH8CH  =  CH.COOH.  In  1871  Geuther  discovered 
liquid  or  iso-crotonic  acid,  and  in  some  works  on  Organic 
Chemistry  this  is  still  given  the  formula  once  ascribed  to  the 
solid  acid;  namely,  CH2  =  CH  —  CH2  —  COOH,  but  it  gives 
acetic  acid  with  alkalies  exactly  as  the  a  acid  does,  and  there- 
fore the  same  reasons  would  hold  good  for  giving  it  also  the 
formula  CH3CH  =  CH  .  COOH.  More  recently  a  true  vinyl- 
acetic  acid  having  the  above  formula  has  been  prepared,  and 
shown  to  be  different  from  either  of  the  crotonic  acids. 

We  are  left  then  with  the  two  crotonic  acids  having  the 
same  structural  formula.  Assuming  that  they  are  geometrical 
isomers,  they  may  be  represented  as  follows :  — 

H         CH8  H         CH3 

\  /  \  / 

p  p 

II      (1)  and  II       (2) 

C  C 

/  \  /  \ 

H         COOH  COOH         H 

The  question  then  arises,  Is  there  any  way  of  deciding 
which  of  these  formulas  should  be  ascribed  to  the  solid  and 
which  to  the  liquid  acid?  Wislicenus  answers  this  question 
in  the  affirmative,  and  draws  his  answer  from  the  reactions 


ILLUSTRATIONS   AND  APPLICATIONS.  55 

of  the  two  /2-chlorcrotonic  acids  with  potassium  hydroxide. 
The  formulas  for  these  two  acids  must  be :  — 

Cl         CH8  CH3         Cl 

\  /  \   / 

c  c 

II      (1)  and  II      (2) 

C  C 

/  \  /  \ 

H         COOH  H         COOH 

Both    of    them    give   with    potassium   hydroxide   tetrolic 
acid :  — 

CH, 

I 

C 

III 

C 

1 

COOH 

but  (1)  might  naturally  be  expected  to  do  this  most  easily. 
By  experiment,  the  /2-chlor-iso-crotonic  acid  is  found  to  give  off 
its  hydrochloric  acid  much  less  easily  than  the  /3-chlor-crotonic 
acid,  therefore  formula  (1)  is  given  by  Wislicenus  to  the 
normal  acid,  and  (2)  to  the  iso-acid.  Correspondingly, 

H         CH3 
\   / 

C 

II 

C 
/  \ 

H         COOH 

• 

should  represent  the  normal  or  solid  crotonic  acid,  and 


56 


THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


CH 


H 


C 


c 

H         COOH 

the  iso  or  liquid  acid. 

Other  methods  of  preparation  of  crotonic  acid,  showing  its 
constitution,  are  the  following :  — 

1.   a-brombutyric  acid  +  KOH 


H 

H        CH3 


H 


CH8 


\ 


+  KOH  =  HBr  + 


H 


\ 


\ 


Br 


COOH 


H 


2.    Distillation  of  /?-oxybutyric  acid, 


OH 


H 


\ 


CH8 


H 


\ 


COOH 


CH3 


=  H20 


C 


H 


\ 


\ 


COOH 


H 


H 


COOH 


It  may  be  interesting  in  this  connection  to  note  that  the 
reaction  between  allyl  cyanide  ^nd  potassium  hydroxide  has 
been  explained  by  the  assumption  that  /3-oxy  butyric  acid  is  first 
formed,  and  then  that  a  further  reaction  takes  place  as  just 
given.  The  principal  fact  in  favor  of  this  assumption  is  that 


ILLUSTRATIONS  AND  APPLICATIONS. 


57 


allyl  cyanide  is  known  under  the  influence  of  hydrochloric 
acid  to  undergo  a  somewhat  analogous  change,  forming 
/?-chlorobutyric  acid,  and  then  by  the  splitting  off  of  hydro- 
chloric acid  to  give  crotonic  acid. 

In  studying  the  reactions  of  the  crotonic  acids  and  compar- 
ing them  with  the  results  to  be  expected  from  the  theories 
under  consideration,  we  are  met  with  the  difficulty  that 
different  investigators  report  different  results  for  the  same 
experiment.  Wislicenus,  for  example,  reports  that  on  addi- 
tion of  chlorine  to  solid  crotonic  acid,  he  obtains  a-/?-dichlor- 
butyric  acid,  and  upon  adding  chlorine  to  the  liquid  crotonic 
acid,  he  obtains  iso-a-/?-dichlorbutyric  acid.  These  results 
agree  perfectly  with  our  theories,  as  is  shown  by  the  follow- 
ing equations :  — 


01 


H 


\ 


CH3 


Cl 


CH3 


\ 


H 


C 


C 


H         CH3 
\  / 
C 

(1)       II  -f  C12          I      =  (by  rotation)      I 

CO  C 

/  \  /|\  /|\ 

H         COOH        H    I    COOH  H    I    COOH 

Cl  Cl 

a-/3-dichlorbutyric  acid. 


CH, 


\ 


H 


CH3 


Cl 


\ 


H 


H 


Cl 


\ 


CH3 


C  C  C 

(2)       II        +  Cla  =        I     =  (by  rotation)      I 
C  C  C 

/  \  /|\  /|\ 

H         COOH        H        COOH  H        COOH 

Cl  Cl 


iso-o-/3-dichlorbutyric  acid. 


58  THE   DEVELOPMENT  OF   STEREO-CHEMISTRY. 

Michael,  however,  states  that  working  with  the  purest 
possible  acids,  he  obtained  from  reaction  (1)  a  mixture  of 
both  a-/2-dichlor  acids,  whereas  in  reaction  (2)  he  obtains 
almost  entirely  the  a-/3-dichlorbutyric  acid  instead  of  the  iso 
acid.  There  are  a  number  of  other  reactions  in  which  Michael 
obtains  mixture  of  the  normal  and  iso  compounds  where 
theory  demands  that  only  one  should  be  formed,  and  in  some 
cases  he  obtains  the  normal  where  the  iso  compound  might 
be  expected,  and  vice  versa.  As  further  illustration  of  this 
discordance  in  views,  we  may  notice  the  reaction  between 
a-/?-dichlorbutyric  acid  and  KOH.  According  to  theory,  the 
reaction  should  take  place  thus:  — 


Cl 


CH8 


\ 


H  CH8         H 

\  / 


c  c 

I         -  HC1  =        II 
C  C 

/l\  /  \ 

H        COOH  Cl         COOH 

Cl  a-chlor-iso-crotonic  acid. 

and  Wislicenus  reports  this  to  be  the  essential  product  as 
found  by  his  experiments,  whereas  Michael  states  that  he 
obtains  a  mixture  of  this  with  the  a-chlor-crotonic  acid. 

Again,  Michael  states  that  the  addition  of  sodium  amalgam 
to  a-brom-iso-crotonic  acid: 


gives  solid  crotonic  acid,  whereas  theory  would  demand  that 
it  should  be  the  corresponding  iso  acid. 


ILLUSTRATIONS  AND  APPLICATIONS.  59 

It  is  evident,  then,  that  the  crotonic  acids  have  not  yet 
met  with  the  full  and  complete  investigation  which  would  be 
necessary  in  order  that  their  reactions  might  properly  be  made 
use  of  either  for  the  defence  or  overthrow  of  any  theory. 
Wislicenus  himself,  in  his  recent  Inaugural  Dissertation, 
concludes  that  the  liquid  acid  commonly  called  iso-crotonic 
is  really  a  mixture  of  the  two  geometrically  isomeric  modifi- 
cations, which  is  formed  in  all  efforts  to  isolate  the  unknown, 
true  iso-crotonic  acid,  on  account  of  the  great  instability  of  the 
latter.  This  may  prove  to  be  the  cause  of  the  varied  results 
obtained  in  working  with  the  crotonic  acids.  At  all  events, 
the  difficulty  of  preparing  perfectly  pure  organic  compounds, 
and  of  separating  and  recognizing  closely  related  and  similar 
products  of  a  reaction,  as  also  the  ready  change  of  a  com- 
pound into  its  geometrical  isomer,  may  be  brought  forward  as 
adequate  reasons  for  the  contradictory  statements  in  regard 
to  the  reactions  of  these  acids.  The  difference  in  conditions 
under  which  the  experiments  are  done  must  also  have  an 
effect  upon  the  results.  It  is  known  that  the  liquid  is  con- 
verted into  the  solid  crotonic  acid  at  170°  to  180°,  and  that 
the  influence  of  light  is  very  noticeable  in  these  reactions,  and 
it  may  be  that  these  two  acids  are  converted  into  one  another 
during  the  course  of  experiments  by  influences  of  whose 
effects  we  are  not  at  present  aware.  This  much  is  certain, 
that  further  experimental  work  is  necessary  before  the  com- 
plete bearing  of  these  reactions  on  Wislicenus's  hypothesis 
can  be  traced.  The  close  relations  between  the  two  crotonic 
acids,  the  ready  transformation  of '  one  into  the  other,  and 
their  similarity  of  reactions  would  seem  to  indicate  that 
they  are  really  geometrical  isorners;  that  is,  bodies  whose 
isomerism  can  best  be  explained  by  van't  HofFs  theories  in 
regard  to  the  relative  positions  of  the  atoms  in  space.  The 


60          THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

only  question  which  is  brought  into  doubt  by  the  conflicting 
reactions  is  in  regard  to  Wislicenus's  supposition  that,  in  the 
formation  of  addition  compounds,  the  atoms  keep  as  nearly 
as  possible  their  original  positions. 

The  third  member  of  the  acrylic  acid  series,  C5H802,  has 
two  well-known  representatives, — angelic  and  tiglic  acids.1 
Angelic  acid  has  been  known  since  1842;  tiglic  acid  was  first 
prepared  in  1865  by  Frankland  and  Duppa,  who  proved  that 
it  was  isomeric  with  angelic  acid.  Since  that  time  the 
isomerism  of  the  two  acids  has  been  a  common  subject  for 
investigation.  Whether  these  two  bodies  are  really  geomet- 
rical isomers,  or  possess  different  structural  formulas,  still 
remains  an  open  question,  but  their  reactions  seem  to  be 
better  explained  by  the  former  assumption  than  by  any 
structural  formulas  which  have  yet  been  assigned  them. 

The  fact  that  tiglic  acid  is  made  by  heating  angelic,  that 
both,  with  potassium  hydroxide,  yield  acetic  and  propionic 
acids,  that  they  yield  the  same  oxidation  products  with  potas- 
sium permanganate,  and  the  same  reduction  products  with 
hydriodic  acid,  with  many  other  similarities  in  chemical 
reactions,  would  indicate  the  closest  similarity  in  their 
structural  formulas. 

The  structure  of  tiglic  acid  is  well  shown  by  its  preparation 
from  a-methyl-/3-oxybutyric  acid :  — 

OH 

CH3    I    H  CH3         H 

\|/  \  / 

C  C 

I  -  H20  =  II 

C  C 

/\\  /  \ 

COOH        CH3  COOH         CH8 

H 

1  Ann.  Chem.  Pharm.  250,  224 ;  Guide  to  Lit.  of  Angelic  and  Tiglic  Acids, 
by  Henry  P.  Talbot,  Ph.  D. ;  Michael's  Untersuchungen  ueber  Alloisomerie. 


ILLUSTRATIONS  AND   APPLICATIONS. 


61 


Assuming  that  these  bodies  are  geometrical  isomers,  the 
above  is  the  formula  generally  assigned  to  tiglic  acid,  and  the 
following  to  angelic  :  — 

CH3         H 
\   / 
C 

II 
C 


\ 


CH3 


COOH 


Both  acids  are  found  to  yield  methylethylacetic  acid  upon 
reduction.  This  is  readily  seen  to  be  in  accordance  with  the 
formulas  given  :  — 


H 


CH3 


H 


OH, 


\ 


\ 


H 


C 


C 


+  H2  = 


(1) 


\ 


GIL 


H 


\ 


CH, 


COOH 


CH 


+  H2  = 


COOH 


CH8        COOH 
H 


H 


H 


\ 


OIL 


I      (2)    By  rotation,  (1)  =  (2) 
G 


CH, 


COOH 


H 


With  hydriodic  acid  two  different  addition  compounds  are 
formed,  differing  slightly  in  physical  properties,  and  similar 
results  are  obtained  with  hydrobromic  acid :  — 


62         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


CH8         H  CHS         H 

\  X  \  X 

C  C 

II         +  HI  =          |      (1) 
C  C 

x  \  /i\ 

CH8         COOH        CH8    |    COOH 

H 


I 

H         CH3  H        CH8 

\  X  \  X 

C1  P 

II          +  HI  =          I      (2) 
C  C 

x  \  x|\ 

CH3         COOH          CH3         COOH 

H 


(1)  and  (2)  are  seen  to  be  geometrical  isomers,  of  which  slight 
difference  might  be  expected. 

These  two  iodine  acids  treated  with  sodium  carbonate  each 
yield  pseudobutylene,  and,  though  there  has  been  some  differ- 
ence of  opinion  as  to  whether  it  is  one  and  the  same  pseudo- 
butylene, or  whether  there  are  two  geometrically  isomeric 
bodies,  Wislicenus  describes  minutely  very  careful  experi- 
ments by  which  he  and  his  pupils  seem  to  have  completely 
proved  the  latter.  This  is  strictly  in  accordance  with  his 
theory  as  shown  by  the  following  equations :  — 


ILLUSTRATIONS   AND   APPLICATIONS. 


63 


I  I 

CH3    I     H  CH3    I     H 

\l/  \rx 

p  c* 

I      +  Na2C03  =      I 

c  c 


CH< 


H 


H 


I 


\ 


\ 


CH8 

L 
\ 

C 

I 
c 

/IN 


H 


\ 


COOH        CH,    I    COONa        CH     I    COONa 


H 


H 


CH8 


CH., 


\ 


r\  r\ 

I      +  Na2C03  =      I 
C  C 


H 


H         CH8 
\  / 
C 

=  Nal  +  CO2  +      II 
C 

/  \ 
CH8         H 


CH3         H 
\  / 
C 

=  Nal  +  C02  +       II 
C 


CH3        COOH         CH3 
H 


\ 


\ 


COONa 


CH 


H 


H 


By  the  addition  of  bromine  to  angelic  and  tiglic  acids, 
Wislicenus  has  succeeded  in  obtaining  two  different  dibro- 
mides,  differing  slightly  in  melting-point,  crystalline  form, 
and  behavior  to  certain  reagents.  In  order  to  obtain  this 
result,  however,  certain  precautions  have  to  be  carefully 
observed;  the  process  must  be  carried  on  at  a  low  tempera- 
ture, with  little  light,  and  care  that  the  substance  to  be 
added  shall  be  in  large  excess.  To  the  two  dibromides, 
Wislicenus  gives  the  following  formula:  — 


64  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

CH8  COOH  H  COOH 

8\  X  \  / 

H-C-C-Br  CH-C-C-Br 


Angelic  dibromide.  Tiglic  dibromide. 

The  formation  of  these  bodies  is  explained  by  the  following 
equations  :  — 

Br  CH3 

CH8          H  CHS    I    H  H         Br 

\  /  \l/  \  / 

c  c  c 

||          +  Br2  =  I      =  (by  rotation)       I 

C  C  C 

/  \  /l\  /l\ 

COOH         CH8  COOH         CHS  COOH    I    CH3 

Br  Br 


Br  H 


CH 


CH8          H  CH3    I    H  Br 

\  /  \l/  \ 

C  C  C 

||          +  Br2  =  |      =  (by  rotation)       1 

C  C  C 

/  \  /l\  /l\ 

CH3         COOH  CH3    I    COOH  CH8        COOH 

Br  Br 

It  should  be  mentioned,  however,  that  Wislicenus's  work 
with  these  acids  has  been  warmly  criticised  by  Fittig,  who 
finds,  for  example,  only  one  pseudobutylene  resulting  from 
the  reactions  in  which  Wislicenus  obtains  two  geometrical 
isomers.  In  several  other  cases  also  he  gets  different  results 
from  Wislicenus,  and  results  which  are  not  as  well  in  har- 
mony with  the  theories  advanced,  so  that  a  study  of  the 
reactions  of  angelic  and  tiglic  acids  leaves  us  in  about  the 
position  in  which  we  were  after  the  study  of  the  crotonic 
acids  j  namely,  with  the  feeling  that  more  work  can  very 


ILLUSTRATIONS  AND  APPLICATIONS.  65 

profitably  be  done  upon  these  bodies  before  we  are  in  a  posi- 
tion to  decide  whether  their  reactions  are  in  entire  accordance 
with  Wislicenus's  theories  or  not. 

The  facts  seem  to  remain  that  in  these  and  many  other 
bodies,  containing  two  doubly  linked  carbon  atoms,  we  have 
cases  of  isomerism  which  cannot  be  explained  by  the  use  of 
structural  formulas.  Many  of  their  reactions  are  in  complete 
accordance  with  van't  Hoff's  and  Wislicenus's  views,  so  that 
this  isomerism  seems  to  be  better  explained  by  the  use  of 
their  theories  than  by  any  other  yet  advanced.  In  some 
cases,  the  reactions  seem  to  be  in  opposition  to  the  theory 
that,  in  forming  addition  products,  the  atoms  retain  as  nearly 
as  possible  their  original  positions.  It  may  be  that  these 
reactions  have  not  been  sufficiently  studied  to  make  sure 
of  their  correctness.  Granting  that  the  adverse  results  as 
obtained  are  correct,  we  must  remember  that  a  body  is  always 
readily  changed  to  its  geometrical  isomer,  so  that,  under  the 
conditions  of  the  experiment,  it  may  be  possible  that  the 
change  takes  place  before  the  addition,  resulting  in  a  product 
in  which  the  atoms  have  shifted  their  position. 

Wislicenus  himself,  in  his  recent  Inaugural  Dissertation, 
acknowledges  that  there  are  cases  in  which  the  abnormal 
addition  product  is  formed  in  larger  quantities  than  the  nor- 
mal, and  that  the  yield  sometimes  consists  almost  entirely 
of  the  abnormal  product.  He  finds  that  the  amount  of  the 
abnormal  addition  product  increases  with  the  temperature, 
intensity  of  light,  and  length  of  duration  of  process,  but  in 
the  pamphlet  under  consideration,  limits  himself  to  the  dis- 
cussion of  the  changes  produced  by  heat. 

He  assumes  that  all  atoms  are  in  a  state  of  oscillation 
around  a  middle  point,  and  that  this  motion  is  increased  by 
heat.  As  in  many  compounds  this  increased  motion  results 

5 


66  THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 

in  decomposition,  so  in  bodies  containing  asymmetric  carbon 
or  two  doubly  linked  atoms  it  may  result  simply  in  a  change 
of  configuration.  For  example,  many  cases  are  known  in 
which  an  active  body  becomes  inactive  under  the  influence  of 
heat,  because  when  the  body  has  reached  the  mean  tempera- 
ture at  which  the  change  to  the  geometrical  isomer  is  pos- 
sible, one  half  the  molecules  are  above  that  temperature  and 
consequently  an  equal  number  of  the  right  and  left  configura- 
tions are  produced. 

Wislicenus  has  determined  the  percentage  amount  of 
change  from  maleic  to  fumaric  acid  under  different  conditions. 
The  quantitative  change  varies  from  25.76%  to  79.69%,  the 
amount  of  fumaric  acid  increasing  with  the  length  of  time 
that  it  is  kept  at  a  fixed  temperature  and  also  with  the  tem- 
perature, but  a  greater  duration  of  time  had  more  effect  on 
the  amount  of  change  than  increased  temperature.  With 
other  compounds,  particularly  the  tolane  dichlorides  and 
dibromides,  he  observed  a  tendency  for  the  two  geometrical 
isomers  to  reach  the  same  final  state  of  equilibrium  at  a  high 
temperature,  whichever  isomer  was  taken  as  the  starting- 
point.  If  the  plane-symmetrical  tolane  dibromide,  for 
example,  be  heated,  the  final  result  is  that  46.56%  of  the 
original  configuration  is  left  unaltered,  and  53.44%  is  changed 
to  the  centre-symmetrical  body;  that  is,  — 

C6H6         Br  C6H5         Br  C6H6         Br 

\  /  \  /  \  / 

C  C  C 

100  parts      II      =  46.56  parts      II      +  53.44  parts      II 

C  C  C 

/  \  /  \  /  \ 

C6Hfi         Br  CeH5         Br  Br         C6H6 

If,  however,  the  centre -symmetrical  body  alone  be  heated, 
the  final  result  contains  48.05%  of  the  plane-symmetrical 


ILLUSTRATIONS   AND   APPLICATIONS.  67 

isomer  and  51.95%  of  the  original  centre-symmetrical  body,  a 
result  which  approximates  the  one  given  above. 

These  experiments  suffice  to  show  the  easy  transformation 
of  a  body  into  its  geometrical  isomer,  the  influence  of  the 
temperature  upon  the  results  which  may  be  expected  in  any 
particular  reaction,  and  some  of  the  difficulties  in  the  way  of 
drawing  definite  conclusions  as  to  the  configurations  of  doubly 
linked  compounds  from  a  study  of  their  chemical  reactions. 


III. 

THE   BENZENE   SERIES. 

THE  preceding  chapters  have  been  devoted  entirely  to  the 
study  of  open  chains.  Quite  recently  von  Baeyer  has 
attempted  the  stereo-chemical  investigation  of  closed  rings, 
the  ultimate  end  of  which  must,  of  course,  be  to  throw  more 
light  upon  the  constitution  of  benzene  and  its  derivatives. 
This  opens  up  afresh  the  whole  interesting  subject  of  the 
constitution  of  benzene,  and  it  may  not  be  out  of  place 
here  to  take  a  brief  review  of  the  work  already  done  in  that 
direction. 

There  are  certain  facts  which  must  be  represented  in  any 
satisfactory  formula  for  benzene, —  namely,  the  equality  of  all 
of  the  carbon  atoms  and,,  thei££orfM  ,tliajmpossibility  of  more 
than  one  mono-substitution  compound;  the  capability  of  for- 
mation of  three  di-substitution  products ;  the  possibility  of 
forming  hexa-addition  products  "ISSTjet  the  fact  that  such 
compounds  are  formed  with  difficulty  and  show  a  tendency  to 
go  back  to  the  simpler  forms;  the  stability  of  benzene,  which 
suggests  a  symmetrical  formula;  and  the  possibility  of  its 
formation  from  acetylene.  These  are  only  a  few  of  the  sim- 
pler facts  which  must  be  represented  in  a  formula  for  ben- 
zene, and  yet  it  is  sufficiently  difficult  to  find  any  formula 
which  will  perfectly  satisfy  these  conditions. 


THE   BENZENE   SERIES. 


69 


In  1865  Kekule l  introduced  the  formula  for  benzene  which 
has  been  in  most  common  use, 


and  which  is  in  fair  accordance  with  most  of  the  facts  men- 
tioned above.  The  symmetry  of  the  molecule,  the  equality  of 
the  six  carbon  atoms,  and  the  possibility  of  formation  of  a 
hexa-addition  product  are  perfectly  represented,  and  the  pos- 
sibility of  its  formation  by  the  condensation  of  three  mole- 
cules of  acetylene  is  in  better  accord  with  this  formula  than 
with  almost  any  other  which  has  ever  been  suggested.  The 
objections  raised  against  it  are  principally  that  it  admits  of 
four  di-substitution  compounds  instead  of  three,  and  that  it 
does  not  explain  the  unwillingness  of  benzene  to  form  addi- 
tion compounds.  On  the  contrary,  since  there  are  three 
ethylenic  bonds,  we  should  expect  an  eagerness  to  form  addi- 
tion compounds,  and  a  tendency  for  the  molecule  to  split  at 
the  points  of  double  linkage.  It  was  at  one  time  suggested 
by  von  Baeyer  2  in  regard  to  this,  that  the  three  double  bonds 
in  a  closed  ring  might  have  a  somewhat  different  g.r»>.iflfl  from 

Ty>>r,t-  fV>Py  wnnlrl  in  an  nppr>  pTiaJr^  f.TiA  tpnr^p.y  ^p  form  addi- 
tion compounds  being  counterbalanced  by  the  gravitation  of 
the  atoms  toward  the  centre  of  the  molecule,  but  this  idea 

1  Bull.  Soc.  Chem.  (2)  3.98;   Ann.  Chem.  Pharra.  137,  129. 

2  Ber.  23,  1272. 


70  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

would  perhaps  be  better  expressed  by  Armstrong's l  modifica- 
tion of  Kekule's  which  he  calls  the  centric  formula:  — 


In  this,  the  energy  is  represented  as  directed  toward  the  cen- 
tre of  the  molecule  instead  of  holding  together  any  two  par- 
ticular atoms. 

In  regard  to  the  objections  urged  against  Kekule's  formula 
with  reference  to  the  di-substitution  products,  while  it  is  true 
that  1  :  2  arid  1  :  6  — 

C1 

//  \ 
«C         C2 

are  not  strictly  identical,  it  seems  natural  to  expect  that  the 
difference  beween  these  two  ortho  compounds  would  be  less 
than  between  the  ortho  and  meta,  or  ortho  and  para,  and  it 
is  quite  conceivable  that  the  differences  might  be  so  slight 
as  to  escape  observation  altogether.  Kekule  himself  has 
endeavored  to  reconcile  his  formula  with  the  existence  of 
only  three  di-substitution  products,  by  the  assumption  that 
the  bonds  indicate  vibrations,  and  that  two  atoms  joined  by 
single  bonds  approach  one  another  twice  as  often  in  a  given 
time  as  two  atoms  with  double  bonds,  that  GI  approaches  C2 
twice,  then  C6  once,  but  then  reverses,  and  approaches  C6 
twice  and  GI  once.  This  makes  a  constantly  shifting  formula, 
being  one  moment 

1  J.  Chern.  Soc.  51,  264. 


THE  BENZENE   SERIES.  71 

C1  C1 

//  \  and  the  next  /  <^ 

6Q  C2  6Q  C2 

The  problem  of  the  constitution  of  benzene  has  also  been 
attacked  from  the  physical  side.  Julius  Thomsen,  whose 
work  will  be  reviewed  more  in  detail  later,  decided  at  one 
time  from  his  thermo-chemical  investigations,  that  the  six 
carbon  atoms  of  benzene  must  be  bound  together  by  nine  sin- 
gle bonds  instead  of  six  single  and  three  double,  and  there- 
fore rejected  Kekule's  formula.  On  the  other  hand  Briihl,1 
working  in  1887,  gave  support  to  Kekule,  deciding  from  opti- 
cal considerations  that  double  bonds  must  be  present. 

Claus's  diagonal  formula,  introduced  in  1867,  differs  from 
the  centric  formula  given  above  only  that  in  this  the  diago- 
nally opposite  carbon  atoms  are  represented  as  bound  together. 


This  claims  the  advantage  over  Kekule's,  in  that  it  shows  the 
possibility  of  only  three  di-substitution  products,  and  it  does 
away  with  the  ethylenic  bonds.  It  has,  however,  certain  dis- 
advantages. It  does  not  seem  in  as  complete  accordance  with 
the  formation  of  benzene  from  acetylene  as  Kekule's  formula 
does,  and  it  leaves  no  explanation  for  the  formula  for  naph- 
thalin.  Moreover,  using  this  formula,  we  have  to  admit  the 
possibility  of  the  formation  of  addition  products  by  the  break 
ing  of  single  bonds. 

1  Ber.  21,  2288;  20,  562. 


72     THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

These  same  objections  apply  to  the  much-discussed  prism 
formula  of  Ladenburg,  introduced  in  1876,  which  represents 
the  six  carbon  atoms  as  lying  in  the  solid  angles  of  a  right- 
angled  prism  on  a  triangular  base.  Another  criticism  which 
has  been  brought  against  this  is  that  it  is  an  unsymmetrical 
figure,  but  if  the  upper  triangle  be  rotated  through  180°  and 
then  projected  on  a  horizontal  plane,  a  star-like  symmetrical 
figure  is  produced.  This  shows  as  well  as  the  other  formulas 
the  possibility  of  only  one  mono-substitution  product  and 
three  di-substitution  compounds.  This  formula  has  met  with 
some  very  strong  support,  and  has  the  prestige  of  being  the 
first  attempt  at  a  stereo-arrangement  for  the  benzene  molecule. 

Only  one  other  formula  need  be  mentioned  here,  the  one 
introduced  by  Dewar  in  1867, 


but  this  seems  to  have  no  essential  advantages  over  Kekule's. 
The  formation  of  benzene  from  acetylene  is  well  represented, 
and  it  is  in  accordance  with  Graebe's  naphthalin  formula; 
but  here  again  the  Ae#&-addition  products  cannot  be  formed 
without  loosing  the  single  bonds,  and,  moreover,  there  are 
apparently  two  different  kinds  of  carbon  atoms,  and  there- 
fore the  possibility  of  two  mono-substitution  compounds  is 
represented. 

In  1880,  Julius  Thomsen *  undertook  to  decide  the  number 
of  single,  double,  and  triple  linkings  in  organic  compounds, 
by  thermo- chemical  investigations.  He  deduced  a  formula 

i  Ber.  13, 1388,  1808,  2166. 


THE  BENZENE   SERIES.  73 

in  which  the  heat  of  formation  of  a  hydro-carbon  is  shown  to 
be  a  function  of  the  number  of  double,  single,  and  triple  link- 
ings.  The  results  as  calculated  from  this  formula  agree 
fairly  well  with  the  observed  heats  of  formation  in  the  case 
of  the  paraffins,  and  also  in  the  case  of  the  defines  if  we 
assume  the  formulas,  as  generally  adopted,  with  one  double 
linking  in  an  open  chain  rather  than  a  closed  ring  with  all 
single  linkings.  Applying  his  formula  to  benzene,  he  found 
that  the  calculated  value  agreed  most  nearly  with  the  observed 
value  if  he  assumed  nine  single  linkings  instead  of  six  single 
and  three  double.  He,  therefore,  at  that  time  accepted 
Clauses  or  Ladenburg's  formula  rather  than  Kekule's  and 
some  later  experiments,1  comparing  the  heat  of  combustion  of 
benzene  with  that  of  dipropargyl  and  of  acetylene  seemed  to 
be  in  accordance  with  this  idea. 

Of  the  two  formulas,  he  preferred  Ladenburg's  to  Clauses 
as  satisfying  the  need  for  an  arrangement  in  space  of  the  six 
carbon  atoms,  but  in  1886  he  writes  that  on  account  of  the 
lack  of  symmetry  of  the  prism  formula  and  its  want  of  agree- 
ment with  certain  chemical  considerations  he  does  not  find 
it  entirely  satisfactory,  and  suggested  instead  an  octahedral 
symbol, 2  since  a  perfectly  symmetrical  arrangement  in  space 
of  six  carbon  atoms  would  lead  to  the  regular  octahedron.8 
According  to  this  theory,  then,  the  six  carbon  atoms  are  regu- 
larly distributed  in  a  spherical  surface,  and  correspond  to  the 
solid  angles  of  a  regular  octahedron.  The  three  axes  of  the 
octahedron  hold  together  three  pairs  of  carbon  atoms.  Each 
atom  is  joined  to  two  neighboring  atoms  through  an  edge,  and 
to  a  third  through  an  axis  of  the  octahedron.  (See  figure, 

1  Ber.  15,  328.  2  Ber.  19,  2944. 

8  Such  a  symbol  had  been  previously  suggested  by  R.  Meyer  in  1882 
(Ber.  15,  1823). 


74 


THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


in  which  the  heavy  lines  indicate  the  linkings  between  the 
atoms) :  — 


V 


This  symbol  fulfils  the  necessary  condition  of  having  all 
the  carbon  atoms  equal,  and  shows  the  possibility  of  having 
three  and  only  three  di-substitution  products.  This  is  shown 
more  plainly  by  the  use  of  the  projection  of  the  octahedron, 
which  is  a  regular  hexagon :  — 


If  now,  of  the  twelve  edges  of  the  octahedron,  those  are 
taken  away  which  are  not  used  in  the  constitution  of  benzene, 
the  following  figure  results :  — 

I 


THE  BENZENE   SERIES. 


75 


This  figure  is  exactly  like  Glaus 's  diagonal  formula  for  ben- 
zene, but  with  a  very  different  idea  behind  it.  This  now  repre- 
sents the  projection  of  a  solid  figure,  and  the  diagonal  bonds 
are  the  axes  of  the  octahedron  —  1:2,  and  1  :  4  differ  in  that 
one  is  a  peripheral,  the  other  an  axial  combination.  Accord- 
ing to  Thomsen,  the  addition  products  result  through  loosing 
of  the  peripheral  bonds,  and  three  such  bonds  can  be  loosed 
without  the  octahedral  form  being  destroyed.  (See  figures.) 


/^A     or  projected  — 


According  to  this,  the  axial  or  diagonal  bonds  cannot  be 
loosed,  since  they  are  the  essential  basis  of  the  octahedral 
form,  whereas  in  Claus's  formula,  the  diagonal  bonds  repre- 
sent energy  in  excess  of  that  necessary  to  hold  the  molecule 
together,  which  can  be  used  in  the  formation  of  addition 
products. 

Thomsen  himself,  however,  soon  brought  one  objection 
against  his  own  formula.  It  was  that  if  this  formula  be  cor- 
rect, benzene  might  be  expected  to  crystallize  in  the  octahe- 
dral instead  of  the  rhombic  system.  And  more  recently 1  he 
has  altogether  discarded  this  formula  together  with  his 
former  views  that  benzene  must  contain  nine  single  bonds. 
He  finds  that  in  the  case  of  trimethylene  the  calculated  heat 
of  combustion  agrees  with  that  observed,  only  on  the  sup- 
position that  trimethylene  contains  three  half -double  bonds. 


1  Zeit.  ph.  Chem.  7,  55. 


76  THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 

He  concludes,  therefore,  that  a  double  bond  consists  of  two 
single  bonds  which  partially  neutralize  each  other,  and  from 
further  experiments,  he  decides  that  in  benzene  there  must 
be  more  than  one  kind  of  linkage  and  that  his  formula  for 
heats  of  formation  can  be  used  only  by  ascribing  to  benzene 
three  double  bonds.  All  objections  to  Kekule's  formula  are 
therefore  withdrawn  by  Thomsen  just  at  present. 

Kekule^s,  Clauses,  Ladenburg's,  Armstrong's,  Dewar's,  and 
Thomson's  are  then  the  principal  formulas  which  have  been 
proposed  for  benzene ;  and  though  none  of  them  are  without 
merit  as  symbols  to  represent  the  properties  of  benzene,  no 
one  of  them  is  so  perfect  and  so  plainly  superior  to  the  rest  as 
to  be  accepted  without  cavil  by  all.  It  is  to  be  noticed  that 
all  of  these  theories  are  entirely  independent  of  the  nature, 
shape,  etc. ,  of  the  carbon  atom,  and  are  concerned  only  with 
the  way  in  which  the  carbon  atoms  are  united. 

The  most  Decent  work  in  connection  with  benzene  has  been 
done  by  von  Baeyer,  who,  beginning  in  1885  with  a  study  of 
the  polyacetylene  derivatives,  considered  next  the  general  sub- 
ject of  ring  formations,  and  finally  turned  his  attention  almost 
entirely  toward  the  aromatic  series,  the  most  important  and 
best  known  of  the  closed  ring  compounds.  Von  Baeyer  has 
prepared  a  number  of  new  acetylene  and  di-acetylene  deriva- 
tives, his  object  being  to  build  up  longer  carbon  chains  and 
study  their  properties. 

Among  such  compounds  prepared,  and  first  described  in  a 
paper  published  in  March,  1885,  is  di-acetylene-dicarbonic- 
acid-ether  C2H6  -  C02  -  C  =  C  -  C  =  C  -  C02  -  C2H6.  This,  he 
says,  explodes  violently  at  about  177°  with  separation  of  a 
very  voluminous  coal,  and  adds  that  this  is,  to  the  best  of  his 
knowledge,  the  first  example  of  a  compound  being  explosive 
which  is  made  up  only  of  carbon,  hydrogen,  and  oxygen.  It 


THE   BENZENE   SERIES.  77 

is  to  explain  the  explosibility  of  the  acetylene  derivatives, 
and  the  greater  explosibility  with  increase  of  number  of  car- 
bon atoms  in  the  chain,  that  he  adopts  what  is  frequently 
referred  to  as  von  Baeyer's  strain  theory.1  He  argues  that, 
to  account  for  the  explosibility  of  such  compounds,  much  heat 
must  be  set  free  in  the  change  to  ordinary  coal  of  acetylenic 
carbon,  and  accounts  for  this  by  the  supposition  that  in  the 
acetylenic  condition  there  is  some  strain  in  the  molecule, 
which  in  the  change  to  coal  is  set  free  in  the  form  of  heat. 

Von  Baeyer's  articles  of  belief  in  regard  to  the  carbon  atom 
are  as  follows:  that  it  has  four  equal  valencies  which  are 
regularly  distributed  in  space  and  correspond  to  the  angles  of 
a  regular  tetrahedron  inscribed  in  a  sphere ;  that  the  carbon 
atoms  can  combine  with  one  another  either  with  one,  two,  or 
three  bonds,  forming  either  open  or  ring-like  closed  chains ; 
and  that  the  four  valencies  of  the  carbon  atoms  work  in  the 
directions  which  bind  the  middle  point  of  the  sphere  with  the 
tetrahedral  angles,  and  which  make  with  one  another  angles 
of  109°  28'.  He  further  adds :  the  direction  of  the  attraction 
can  undergo  a  deviation  which  has  as  its  consequence  a  strain 
increasing  with  the  amount  of  the  deviation. 

To  illustrate  this  last  point  he  refers  to  the  common  models 
in  which  the  carbon  atom  is  represented  as  a  ball  from  which 
wires  project,  making  with  each  other  angles  of  109°  28'.  If 
such  models  be  joined  together  they  will  form  either  a  zigzag 
line,  or  a  closed  ring  of  five  atoms.  If  we  wish  to  make  with 
them  a  ring  of  more  or  less  than  five  atoms,  the  wires  must  be 
bent  from  their  original  positions,  inducing  a  kind  of  strain, 
and  something  entirely  analogous  to  this  von  Baeyer  conceives 
to  take  place  in  the  carbon  atoms  themselves. 

As  an  illustration  of  this  idea  when  applied  to  carbon  com- 

1  Ber.  18,  2277 ;  23,  1272. 


78          THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

pounds  he  considers  the  rings  built  up  of  several  methylene 
groups.  Of  these  the  simplest  is  ethylene.  To  produce  this 
from  ethane,  two  of  the  bonds  must  be  bent  until  they  are 
parallel.  Supposing  both  bonds  bent  equally,  each  must  be 
turned  through  an  angle  |  (109°  28')  =  54°  44'.  To  form 
trimethylene,  which  may  be  pictured  as  an  equilateral  trian- 
gle, the  angle  must  become  60°,  and  each  bond  must  be  turned 
through  £  (109°  28' —  60°).  In  tetramethylene  the  devia- 
tion will  be  represented  by  £  (109°  28'  —  90°).  Pentamethy- 
lene  may  be  represented  as  a  regular  pentagon  whose  angle  is 
108°;  therefore  the  amount  of  deviation  necessary  is  only 
}  (109°  28'  —  108°)  =  44'.  With  hexamethylene,  the  deriva- 
tion becomes  %  (109°  28'  —120°).  These  facts  are  shown  at  a 
glance  by  the  accompanying  figures,  the  angle  of  deviation 
from  the  normal  direction  of  the  valence  being  given  in  each 
case: — 


CHa 

CHa 

CH2=CHa 

II 

/  \ 

1           1 

CHa 

CH2  CH2 

CH2  =  CH2 

540  44' 

240  44/ 

9°  34' 

CH2  CH2 

/     ^  /     \ 

CH2  Co2  CH2  CH2 

II  II 

CH2 CHg  OHj  C/Hj 

\     / 
CH2 
0°  44'  —5°  16' 

From  these  formulas,  it  is  seen  that  the  greatest  "  strain  " 
comes  in  the  ethylene,  and  this  agrees  well  with  fact  in  that 
this  is  the  one  whichjsjnost  ready  to  form  addition  com- 
pounds, being  readily  attacked  by  hydrobromic  acid,  bromine, 


THE   BENZENE  SERIES.  79 

and  even  iodine.  Trimethylene  is  attacked  only  by  bromine, 
and  tetra  and  hexamethylene  not  at  all,  or  only  with  great 
difficulty  by  any  of  these  reagents. 

It  would  follow  from  these  ideas  that  triply  linked  com- 
pounds should  be  more  unstable  than  doubly  linked,  since  the 
larger  the  number  of  bonds  bent  from  their  normal  position, 
the  greater  the  strain  would  be;  and  this  view  meets  with 
some  support  from  Thomsen,  who  has  found  from  calorimetric 
measurements  that  by  the  transformation  of  a  double  linkage 
into  a  triple,  there  is  an  essential  lessening  of  the  stability 
of  the  system  under  consideration. 

Adopting  these  view«,  wfi  ghni]1d  ftYp°^  five-atom  carbon  , 
rings  to  be  formed  more  readily  and  be  more  stable  than  the 
rings  composed  of  six  atoms,  whereas" the  former  are  found 
only  seldom  and  in "  complicated  compounds.  Von  Baeyer 
remarks  in  tills  connection  that  the  objection  is  not  of  great 
weight,  because  the  six-atom  ring  is  found  principally  in  the 
benzenes  which  are  poorer  in  hydrogen,  and  that  under 
exactly  similar  circumstances,  it  is  quite  possible  that  penta- 
methylene  would  be  formed  a  little  more  easily  and  be  a  little 
more  stable  than  the  hexamethylene.  Victor  Meyer  l  thinks 
that  a  possible  explanation  may  be  found  in  the  greater  sym- 
metry of  the  six-atom  ring,  which  is  conducive  to  stability, 
and  calls  attention  to  the  fact  that  the  angle  of  deviation  in 
hexamethylene,  though  larger  than  that  in  the  case  of  penta- 
methylene,  is  still  quite  small. 

To  sum  up  briefly  this  portion  of  von  "Rap.yftr'g  wr>rTr;  he 
adopts  van't  Hoff's  idea  of  thn  Tnleianion  of  fhr  carbon  atoms 
working.. in. .~£he— directions  of "the"~ angles  of  a  tetrahedron, 
assumes  that  these  valencies  can  be  bent  from  their  normal 
position  producing  a  strain,  that  the  greater  this  strain  the 

i  Ber.  23,  580. 


80  THE   DEVELOPMENT   OF  STEREO-CHEMISTKY. 


tj  thnt  if  the  carbon  atoms  be 


arranged  in  a  plane.  frES-op  six  ouch-  atoms  will  form  a  closed 
ring  with  no  or  very  slight  deviation  from  the  normal  direc- 
tion of  the  valencies,  and  that  such  bodies  may,  therefore,  be 
expected  to  be  stable.  Since  benzene  and  its  derivatives  are 
stable  bodies  containing  rings  of  six  carbon'  atoms,  he  con- 
cludes that  in  these  bodies  the  centres  of  gravity  of  the 
carbon  atoms  lie  in  a  plane  instead  of  being  distributed  in 
space. 

All  of  the  later  work  /upon  the  aromatic  series  is  based 
upon  this  last  conclusion,  although  von  Baeyer's  strain  theory 
is  open  to  some  criticism.  That  theory  adapts  itself  well  to 
wooden  models  with  wire  valencies,  but  until  we  have  a  more 
definite  idea  of  what  the  valencies  of  an  atom  really  are,  it  is 
difficult  to  get  ,any  clear  idea  of  what  this  "strain  "  may  be. 
It  will  be  noticed  that  the  strain  hypothesis  originated  in  the 
explosibility  of  the  acetylene  derivatives;  and  Victor  Meyer1 
calls  attention  to  the  fact  that  silver  oxalate  is  very  explosive, 
although  the  conditions  considered  necessary  by  von  Baeyer 
in  his  investigations  are  altogether  lacking.  He  also  thinks 
the  fact  that  acetylene  is  the  only  hydrocarbon  formed  by  the 
electric  spark  from  carbon  in  an  atmosphere  of  hydrogens 
shows  a  stability  of  acetylene  incompatible  with  the  exist- 
ence of  the  great  strain  which  von  Baeyer's  views  would 
demand. 

Whether  von  Baeyer's  work  be  accepted  in  its  entirety  orN 
not,  it  must  certainly  be  acknowledged  that  it  has  proved 
very  fruitful.  Having  arrived  at  the  conclusion  that  the  six 
carbon  atoms  must  He  in  a  plane,  he  has  done  many  experi- 
ments in  the  endeavor  to  reach  some  basis  of  decision  between 
the  plane  formulas  already  mentioned. 

i  Ber.  23,  581. 


THE  BENZENE  SERIES. 


81 


A  part  of  his  earlier  work  in  this  direction l  consisted  in  the 
proof  that  hexamethylene  and  hexahydrobenzole  were  really 
identical.  This  proof  consisted  in  the  transformation  of 
dioxyterephthalic  acid,  by  reduction,  into  succinylo-succinic 
acid,  which  is  a  hexamethylene  derivative.  Since  the  dioxy- 
terephthalic acid  is  a  para  compound,  the  reaction,  using 
Kekule's  formula,  is  represented  by  the  following  equation : 


CH 

II 
COH 


COOC2H6 

I 

c 
% 

COH 

I       + 
CH 


H2  ==' 


COOC2H, 
I 

CH 

/  \ 

CH2        CO 

I 

CO 
\  / 
CH 


CH 


c 

I  I 

COOC2H5  COOC2H5 

In  this  reaction  there  is  evidently  an  addition  of  two 
hydrogen  atoms  and  a  change  of  position  of  two  others,  and 
two  para  positions  are  retained.  Von  Baeyer  makes  use  of 
this  reaction  to  prove  the  prism  formula  untenable.  Using 
Ladenburg's  formula,  this  same  reaction  must  be  represented 
as  follows :  — 


C 

( 

?OOC2H5 

i 

\ 

COOC2H5 
1 
CH 

/  \ 

CH- 

—COH 

CH2 

CO 

1 

1     +  H2  = 

1 

1 

CH— 

—  COH* 

CO 

CH2 

\ 

/ 

\ 

/ 

C 

I 

COOC2H5 


CH 
I 
COOC2H5 


1  Annalen  der  Chem.  245,  103. 
6 


82  THE    DEVELOPMENT   OF   STEREO-CHEMISTRY. 

The  reaction  expressed  in  this  way  involves  a  change  in 
position  of  the  oxygen  atom  and  seems  somewhat  complicated 
and  improbable.  Moreover,  von  Baeyer  argues  against  the 
prism  formula  as  follows :  The  prism  consists  of  two  trian- 
gles held  together  by  three  para  edges.  In  order  to  get  a 
hexamethylene  from  this,  two  of  the  triangular  bonds  and 
one  of  the  para  edges  must  be  broken ;  for  if  we  endeavor  to 
break  two  para  edges ,  we  obtain,  — 


which  would  represent  a  ditrimethylene.  In  the  above  trans- 
formation, however,  two  para  positions  are  retained;  but  this 
would  demand  the  breaking  of  the  corresponding  para  edges 
in  order  that  the  hydrogen  atoms  might  be  substituted  in 
those  places,  which  has  just  been  shown  to  be  impossible.  He 
therefore  concludes  that  only  Kekule's  and  Claus's  formulas 
need  be  taken  into  consideration. 

Ladenburg,  however,  does  not  submit  quietly  to  being  thus 
put  out  of  the  contest,  but  protests  1  that  von  Baeyer  gives 
evidence  against  the  prism  formula  and  new  evidence  in  favor 
of  the  hexagon  without  bringing  any  arguments  against  the 
former  evidence  in  favor  of  the  prism,  and  accordingly  that 
both  formulas  are  possible.  He  still  holds  to  his  opinion  that 
the  prism  formula  is  the  only  one  which  can  give  a  clear  idea 
of  the  benzene  substitution  products,  while  agreeing  that 
Kekule's  formula  is  better  adapted  to  explain  certain  other 
reactions.  He  therefore  concludes  that  the  assumption  which 
is  in  best  accordance  with  facts  is  that  the  benzene  nucleus 

i  Ber.20  62. 


THE   BENZENE   SERIES.  83 

can  exist  in  two  forms, — a  more  stable  form  corresponding 
to  the  prism,  and  a  less  stable  form  corresponding  to  the  hex- 
agonal formula. 

Although  the  prism  formula  was  rejected  by  von  Baeyer,  he 
also,  some  years  later,  came  to  the  conclusion  that  the  exact 
configuration  of  the  nucleus  might  be  different  in  different  ben- 
zene derivatives.  His  work  leading  to  this  decision  is  so  inter- 
esting and  important  that  it  seems  well  worth  while  to  review 
it  briefly,  whatever  may  be  considered  the  definite  outcome 
of  his  studies.  Having  come  to  the  conclusion  that  benzene 
consists  of  six  CH  groups,  with  the  centres  of  gravity  of  the 
six  carbon  atoms  lying  in  one  plane,  it  was  with  the  ultimate 
object  of  determining  how  these  groups  were  joined  together 
that  von  Baeyer  undertook  his  work  with  the  reduction  of  the 
phthalic  acids.  The  comprehensive  scope  and  the  value  of 
his  work  in  this  direction,  leading  as  it  did  to  some  interest- 
ing illustrations  of  geometrical  isomerism  in  the  aromatic 
series,  cannot  be  praised  too  highly  even  if  it  does  not  lead 
positively  to  a  satisfactory  formula  for  benzene. 

When  terephthalic  acid  is  acted  upon  by  sodium  amalgam, 
two  hydrogen  atoms  are  added  in  the  cold,  when  heated  four 
atoms  are  added,  and  with  great  difficulty  six  atoms  of 
hydrogen  can  be  added,  forming  respectively  di-,  tetra-,  and 
hexahydroterephthalic  acids.  The  last  of  these  can  better 
be  prepared  by  the  reduction  of  the  hydrobromide  of  tetra- 
hydroterephthalic  acid,  and  has  the  constitution  represented 
by  the  following  symbol :  — 


84  THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


This  substance  exists  in  two  geometrically  isomeric  forms 
and  acts  in  all  particulars  like  a  saturated  body.  It  is 
not  attacked  by  potassium  permanganate  in  the  cold;  when 
heated,  bromine  acts  upon  it  and  is  substituted  in  the  a  posi- 
tion. This  last  product  is  isomeric  with  the  addition  product 
formed  by  adding  hydrobromic  acid  to  the  tetrahydrotereph- 
thalic  acid.  These  two  isomers  differ  in  properties,  but  by 
addition  of  alcoholic  potash  to  both,  one  and  the  same  tetra- 
hydroterephthalic  acid  results,  and  this  is  identical  with  the 
acid  produced  by  direct  reduction  of  terephthalic  acid. 

These  reactions  show  that  in  the  tetra-acid  produced  by  the 
reduction  of  terephthalic  acid,  the  double  bond  must  be  con- 
nected with  the  carbon  attached  to  carboxyl,  and  von  Baeyer 
has  proved  by  three  different  reactions  that  this  double  bond 
must  lie  between  two  adjacent  carbon  atoms.  Only  one  reac- 
tion in  support  of  this  idea  need  be  given  here.  The  di-addi- 
tion  product  formed  by  adding  bromine  to  tetrahydrophthalic 
acid  has  been  converted  into  a  product  identical  with  tetra- 
brompyrocatechin.  This  would  indicate  that  the  extra  bonds 
lay  in  the  ortho  position.  Accordingly,  the  formula  for  tetra- 
hydroterephthalic  acid  is  written :  — 


THE  BENZENE  SERIES,  85 

COOH 
I 
C 

/^ 
CH2         CH 

I  I 
CH2         CH2 

\   / 
CH 
I 
COOH 

By  somewhat  similar  methods  he  deduces  the  formula  for 
the  dihydroterephthalic  acid  formed  by  reduction  to  be ;  — 

COOH 
I 
C 

/  ^ 

CH         CH 

II  I 
CH         CH2 

\  / 

C 

/  \ 
H        COOH 

These  formulas  are  in  accordance  with  facts  in  that  the  di- 
and  tetra-hydro  acids  act  exactly  like  the  acids  of  the  unsat- 
urated  series  with  ethylenic  bonds,  taking  up  bromine 
instantly  under  the  same  conditions  as  the  cinnamic  acids. 

The  question  now  is,  can  we  go  back  from  these  formulas 
to  the  formula  for  terephthalic  acid,  and  thus  to  the  formula 
for  benzene.  Adopting  the  formula  for  the  di-addition 
product,  that  one  which  lies  nearest  to  the  terephthalic  acid, 
the  simplest  supposition  would  be  that  Kekule's  formula  was 
correct ;  but  here  we  are  met  again  by  the  old  objection  that 
terephthalic  acid  and  benzene  do  not  act  like  unsaturated 


86 


THE  DEVELOPMENT    OF    STEREO-CHEMISTRY. 


bodies,  as  might  be  expected  if  they  had  three  ethylenic 
bonds.  On  the  other  hand,  if  the  diagonal  formula  be 
adopted,  we  must  suppose  that  the  first  addition  is  made  by 
the  breaking  of  a  single  linkage,  and  that  straightway  the 
other  two  diagonal  bonds  go  over  into  ethylenic.  These  ideas 
would  be  as  well,  and  perhaps  better,  represented  by  the  cen- 
tric formula,  and  von  Baeyer  was  at  first  inclined  to  accept 
this,  rejecitng  Kekule's  principally  on  the  ground  that  the 
benzene  derivatives  are  stable  toward  potassium  permanganate; 
whereas  double  bonds,  either  in  open  or  closed  rings,  he  sup- 
posed to  be  instantly  attacked  by  that  reagent.  In  1889, 
however,  he  reversed  that  decision,1  since  it  had  been  shown 
in  the  mean  time  that  under  some  circumstances  a  double  bond 
in  a  ring  formation  could  resist  this  action.  Stilbene 


CH 


-CH  =  CH— <\C 


CH 


CH 


is   immediately  oxidized  by  potassium  permanganate  in  the 
cold,  but  the  similarly  constituted  phenanthrene 


VCH 


CH 


CH 


is  not  attacked. 


Annalen  der  Chem.  251,  256. 


THE    BENZENE    SERIES.  87 

Since  here  is  a  body  containing  double  bonds  which  is  not 
affected  by  potassium  permanganate,  the  fact  that  benzene 
resists  that  reagent  is  no  proof  that  it  has  not  double  bonds. 
On  the  supposition  that  the  three  double  bonds  in  the  closed 
rings  have  a  somewhat  different  nature  from  that  in  the  open 
chain,  the  internal  force  of  gravitation  neutralizing  the  ordi- 
nary tendency  to  instability,  he  concludes  that  the  reactions 
of  terephthalic  acid  are  explained  equally  well  by  Kekule's 
and  by  the  centric  formula. 

In  an  address  made  in  1890  at  the  festival  in  honor  of  the 
25th  anniversary  of  Kekule's  most  important  discoveries, 
von  Baeyer  l  announces  his  conclusion  that  the  valence  in  the 
benzene  molecule  itself  can  no  more  be  expressed  than  that 
in  a  molecule  of  nitrogen ;  it  is ,  like  the  nitrogen  molecule, 
a  stable  whole,  and  its  free  bonds  only  make  their  appearance 
when  the  structure  of  the  kernel  is  disturbed  at  some  point. 
The  aromatic  series  may  be  divided  into  groups  differing 
somewhat  as  to  the  way  in  which  the  carbon  atoms  are  held 
together.  The  most  stable  compounds  are  benzene  and  its 
immediate  derivatives,  the  carboxylic  acids,  etc.  These 
appear  as  the  "  Ideal  "  in  which  the  CH  groups  are  so  firmly 
bound  together  that  the  valence  of  the  carbon  may  be  con- 
sidered as  three  rather  than  four,  the  double  bonds  appearing 
only  under  certain  circumstances.  The  loosest  benzene  com- 
pound is  phloroglucin,  in  which  there  are  undoubtedly  three 
double  bonds,  which  are  only  a  little  more  stable  than  those 
in  bodies  of  the  fatty  series.  Bodies  such  as  naphthalin  and 
phenanthrene  take  a  medium  position,  since  they  add  halogens, 
but  are  stable  against  potassium  permanganate.  The  two 
extreme  limits  of  the  benzene  derivatives  may  then  be  ex- 
pressed by  Kekuld's  and  the  centric  formulas.  Von  Baeyer 

1  Ber.  23,  1272. 


88          THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

concluded  at  that  time  that  the  nature  of  the  benzene  ring 
in  any  derivative  may  correspond  to  a  state  lying  midway 
between  these  two,  and  therefore  that  Kekule's  formula  can 
well  be  retained  for  common  use. 

Von  Baeyer's  object  in  all  his  work  was,  as  he  has  stated, 
not  to  prove  or  disprove  any  particular  formula  for  benzene, 
but  in  so  far  as  possible  to  arrive  at  the  truth;  therefore  he  is 
ready  to  change  his  views  when  new  discoveries  seem  to  make 
it  necessary,  and  in  1892, l  on  the  basis  of  newly  discovered 
facts,  he  gives  the  preference  to  Claus's  diagonal  formula. 
In  the  paper  in  which  he  endeavors  to  prove  that  this  formula 
is  in  better  accordance  with  observed  facts  than  any  of  the 
others,  he  starts  out  with  the  decision  that  no  conclusions 
can  be  drawn  as  to  the  constitution  of  benzene  from  the  reduc- 
tion of  a  benzene  derivative.  This  decision  is  forced  upon 
him  by  observations  which  show  a  serious  mutability  of  the 
double  bonds,  and  an  addition  of  hydrogen  where  it  would 
least  be  expected.  He  finds,  for  example,  that2  A1'8  dihydro- 
terephthalic  acid  gives  by  reduction  A2  tetrahydroterephthalic 
acid,  according  to  the  following  equation:  — 

COOH  H         COOH 

I  \  / 

(1)C  C 

CH2         C2H  H2C(6>      2»CH 

m  I  U      +  Ha  =        I 

per  ripr  in  rv> 

v'.n.jj  v^n  jn2v/(«) 


C  C 

I  /  \ 

COOH  H         COOH 

1  Lieb.  Ann.  der  Chem.  269,  145. 

2  The  numerals  indicate  the  carbon  atoms  which  are  connected  by  double 
bonds  with  the  next  carbon  atom,  going  around  the  ring  in  the  direction  of 
the  hands  of  a  watch. 


THE  BENZENE  SERIES.  89 

Other  observations  of  a  similar  kind  led  him  to  conclude 
that  the  hydrogen  atoms  wandered  sadly,  and  that  one  could 
not  draw  any  inferences  from  the  constitution  of  a  reduction 
product  as  to  the  constitution  of  the  original  body.  Accept- 
ing these  observations  as  correct,  we  shall  have  to  conclude 
that  his  work  with  the  phthalic  and  terephthalic  acids, 
although  extremely  valuable  on  account  of  the  new  isomers 
described,  are  not  of  importance  in  connection  with  the  use 
to  which  he  hoped  to  be  able  to  put  them ;  namely,  to  discover 
the  constitution  of  benzene. 

Von  Baeyer,  however,  affirms  that  the  reverse  action,  the 
building  up  of  the  benzene  nucleus  from  a  hydro  derivative, 
can  furnish  a  clew  to  the  nature  of  benzene,  and  concludes  that 
the  facts  from  this  standpoint  are  in  favor  of  Glaus 's  formula. 
Only  a  very  brief  outline  of  the  observations  which  led  him 
to  this  conclusion  can  be  given  here.  He  points  out  that  the 
three  following  dihydrophthalic  acids  yield  by  oxidation 
benzoic  acid  and  carbon  dioxide:  — 

H         COOH  H         COOH  COOH 

\   /  \  /  I 

OHO  C 

/(1)\  /  /   \  #  \ 

CH(6>     (2)0  -  COOH  CH2       (2)C  -  COOH    wCH       «C  .  COOH 

II  I  I  II          and  I  II 

CHw     wCH  CH         CH  CH2        CH 

CH  C4H  CHa 

A8,6  A2,4  AV 

The  A8-5  acid  undergoes  this  reaction  with  the  greatest  ease, 
even  with  very  weak  oxidizing  agents.  Von  Baeyer's  expla- 
nation is  that  the  withdrawal  of  hydrogen  sets  the  a  carbon 
into  violent  vibration,  whereby  the  carboxyl  is  separated  off, 
and  the  same  explanation  is  applied  to  A2'4;  but  in  A2'6,  the 


90 


THE  DEVELOPMENT    OF    STEREO-CHEMISTRY. 


hydrogen,  instead  of  being  taken  from  the  a  carbon,  is  taken 
from  the  one  in  the  para  position,  and  the  results  are  the 
same  as  in  the  other  cases.  Here,  then,  the  same  effect  is 
produced  whether  the  hydrogen  be  broken  off  from  the  car- 
bon atom  bearing  carboxyl  or  from  the  diagonally  opposite 
carbon.  This  would  be  explained  by  Claus's  formula  for 
benzoic  acid,  and  by  no  other. 

According  to  this  explanation,  when  the  hydrogen  atoms 
are  set  loose  from  a  carbon  atom,  the  latter  is  set  in  violent 
vibration.  If  this  carbon  is  found  in  the  para  position  rela- 
tively to  the  carbon-bearing  carboxyl,  it  combines  with  it 
and  the  ring  goes  over  into  benzoic  acid;  but  it  is  noticeable 
that  the  same  violent  vibration  of  the  carbon  in  the  ortho 
position  does  not  have  this  effect.  When  A1>4  dihydrophthalic 
acid,  for  example,  is  oxidized,  the  hydrogen  is  taken  away 
from  the  carbon  atoms  (3)  and  (6);  but  this  change  cannot 
cause  the  separation  of  carbon  dioxide,  and  phthalic  acid 
results,  according  to  the  following  equation :  — 


COOH 

I 

C 


COOH 
I 


H\c 

H/? 
H05) 


(2>C-COOH 

I 


I 

H 


H 

H 


+  O  =  H20  + 


HC 
HC, 


/LVcOOH 


This  is  the  reaction  which  might  be  expected  adopting 
Claus's  formula.  The  two  carbon  atoms  in  the  para  position 
are  set  in  violent  vibration,  combine  with  one  another,  and 
then  the  two  ethylenic  bonds  also  go  over  into  para  linkage. 


THE  BENZENE   SERIES.  91 

In  this  same  article,  von  Baeyer  brings  forward  another 
argument  against  Kekule^s  formula.  This,  he  says,  would 
demand  two  phthalic  acids,  —  one  in  which  the  two  carboxyl- 
beariiig  carbon  atoms  are  joined  together  by  double  bonds, 
and  another  in  which  they  are  singly  linked;  but  two  differ- 
ent bodies  have  never  been  found.  The  one  in  which  the 
carbons  are  connected  by  single  bonds  might  be  expected  by 
taking  away  hydrogen  from  A2'6  dihydrophthalic  acid;  but 
instead,  as  has  been  seen,  ben  zoic  acid  is  formed,  while  that 
acid  which  has  a  double  bond  between  the  a  carbon  atoms  pro- 
duced phthalic  acid. 

The  reactions  described  above  are  certainly  indications  in 
favor  of  Clauses  formula;  but  without  further  evidence  it 
would  seem  that  they  should  not  be  accepted  as  proof  of  direct 
linkage  between  para  atoms  in  all  aromatic  compounds,  since 
other  reactions  have  been  observed  which  show,  with  perhaps 
equal  force,  the  improbability  of  such  linkage.1  Briihl's 
study  of  the  optical  and  physical  properties  of  organic  bodies 
has  led  him  to  the  conclusion  that  benzene  contains  three 
ethylenic  bonds,  on  the  ground  that  the  change  in  these  prop- 
erties in  passing  from  benzene  to  dihydrobenzene  is  not  suffi- 
ciently marked  to  warrant  the  supposition  that  there  is  a  great 
difference  in  the  molecular  structure  of  these  two  bodies.2 

A  careful  study  of  all  the  work  done  in  connection  with  the 
constitution  of  benzene  seems  to  leave  us  with  no  entirely 
satisfactory  outlook,  although  a  large  amount  of  valuable 
material  has  been  collected  to  aid  in  elucidating  the  problem, 
which,  it  may  be  hoped,  will  be  solved  at  some  not-far-distant 
date.  Meanwhile,  there  seem  to  be  many  reasons,  in  the 
present  state  of  our  knowledge,  for  giving  the  preference  to 

1  Am.  Chem.  Jour.  12,  463 ;  13,  422. 

2  J.  pr.  Chem.  [2],  49,  201 ;   Ber.  27,  1065. 


92          THE  DEVELOPMENT  OF   STEKEO-CHEMISTRY. 

the  centric  formula,  as  expressing  the  reserve  energy  of  the 
benzene  molecule  without  giving  too  definite  a  location  to 
that  energy.  If  further  experiments  should  give  more  con- 
clusive evidence  of  direct  linkage  between  para  atoms,  then  it 
would  be  necessary  to  localize  that  energy  and  change  from 
the  centric  to  the  diagonal  formula.  Kekule's  formula  is 
open  to  the  very  serious  objection  of  showing  ethylenic  bonds 
which  in  other  compounds  have  a  definite  meaning  which 
they  do  not  possess  in  the  benzene  nucleus.  Von  Baeyer's 
suggestion  that  the  active  power  of  these  double  bonds  may  be 
partially  lost  in  a  closed  ring,  being  neutralized  by  the  attrac- 
tion toward  the  centre  of  the  atoms  composing  the  ring,  seems 
a  practical  abandonment  of  Kekule's  for  the  centric  formula. 

It  will  be  readily  seen  that  we  are  not  in  very  good  condi- 
tion at  present  to  make  any  suppositions  in  regard  to  the  three- 
dimension  configuration  of  the  benzene  molecule.  All  of  the 
work  thus  far  done  has  been  based  on  the  supposition  that  the 
centres  of  gravity  of  the  carbon  atoms  composing  the  benzene 
ring  lie  in  a  plane.  Von  Baeyer  says  that  the  material  is  far 
too  meagre  at  present  for  any  conclusions  to  be  drawn  in 
regard  to  the  actual  arrangement  in  space  of  the  benzene  mol- 
ecule, and  others  protest  against  a  too  hasty  effort  to  apply 
three-dimension  formulas  to  the  aromatic  series. 

The  first  contribution  to  our  knowledge  of  geometrical 
isomerism  in  the  benzene  series  was  made  by  yan't  Hoff  in 
1875,  when  he  pointed  out  that  in  hydromellitic  and  isohy- 
dromellitic  acids  we  havertwo  bodies  whogfiJteemeiimii  must 
rest  on  a  difference  of  space  configuration  of  their  molecules. 

Von  Baeyer,  in  his  studyof  the  hydrogen  derivatives  of  the 
phthalic  acids,  observed  that  there  were  two  isomeric  hexa- 
addition  products.  Each  can  be  readily  converted  into  the 
other ;  but  they  differ  in  solubility  and  other  physical  proper- 


THE  BENZENE  SERIES. 


93 


ties  in  a  way  entirely  comparable  to  the  geometrical  isome- 
rism  noticed  in  olefine  derivatives.  Von  Baeyer  explains  this 
on  the  assumption  that  the  carbon  atom  has  its  valencies  in 
the  directions  of  the  angles  of  a  tetrahedron,  and  that  in  the 
benzene  nucleus  the  six  carbon  atoms  are  found  with  their 
centres  of  gravity  in  a  horizontal  plane.  In  hexamethylene, 
then,  the  two  hydrogen  atoms  attached  to  each  carbon  will 
lie  in  a  plane  perpendicular  to  the  horizontal  plane,  and  the 
twelve  hydrogen  atoms  will  be  equally  distributed  in  two 
planes,  one  above  and  the  other  below  the  plane  containing 
the  six  carbon  atoms.  If  now  one  hydrogen  atom  be  removed 
from  each  of  two  carbon  atoms  in  the  para  position  to  each 
other  and  be  replaced  by  carboxyl,  the  result  will  be  hexahy- 
droterephthalic  acid,  and  we  may  have  two  isomeric  bodies 
according  to  whether  both  carboxyls  are  in  the  same  or  in 
different  planes.  As  well  as  this  can  be  shown  on  a  plane 
surface,  the  two  geometrical  isomers  would  be  represented  as 
follows :  — 

H         CH2  -  CH2         H 

\     /  (2)  (3)        \     / 

Cd)  (4)0 


X      X    (6)  (5) 

COOH)       CHs-CH 


and 


2  —  ^AA2 

(1) 


H         CH2-CB2         COOH 

/   (2)  (3)       \     / 

Cd)  (4)C 


(6)  (5) 

CH2-CH2 

(2) 


* 

V 


Of  these  (1)  is  called  the  cis  acid,  or  the  maleinoid,  on 

account  of  its  resemblance  to  maleic  acid-;- 

/^ 

-     * 


OF  THE 

(UNIVERSITY 

OF  _ 


94  THE   DEVELOPMENT  OF    STEREO-CHEMISTRY. 

H  H 

\  X 

C^C 

x          \ 

COOH  "  COOH 

and  (2)  the  trans,  or  fumaroid,  since  the  carboxyls  are  on 
opposite  sides,  as  in  fumaric  acid :  — 

H  COOH 

\  / 

C=C 
/  \ 

COOH  H 

The  general  characteristics  of  these  different  acid^jjflira- 
spond  also  to  the  differences  in  properties  of  maleic  and  fumaric 
acids.  The  cis  acids  are  more  easily  soluble,  have  a  lower 
melting-point,  and  are  stronger  acids  than  their  trans  iso- 
mers,  thus  resembling  maleic  acid ;  but  the  trans  acids,  like 
fumaric  acid,  are  more  stable,  as  is  shown  by  the  fact  that 
they  may  be  produced  by  the  direct  transformation  of  the  cis 
acids ;  as,  for  example,  by  heating  with  hydrochloric  acid. 

In  the  above  formulas,  the  carbon  atoms  numbered  (1)  and 
(4)  are  asymmetric,  because  if  (1)  be  moved  from  right  to  left 
around  to  (4)  the  carboxyl  will  be  on  the  right  of  the  carbon 
atom,  whereas,  if  movecf  to  the  same  position  from  left  to 
right,  the  carboxyl  will  be  on  the  left.  Hence  it  follows  that 
the  two  affinities  of  (1)  are  not  bound  in  the  same  way,  and 
that  it  is,  therefore,  asymmetric.] This  is  called  relative 
asymmetry,  and  it  is  characterized  by*  the  fact  that  the  asym- 
metry of  one  of  the  two  carbon  atoms  is  compelled  by  that  of  the 
other,  for  if  the  carboxyl  of  (4)  be  replaced  by  hydrogen,  the 
asymmetry  of  (4)  and  (1)  vanishes  simultaneously.  This  kind 
of  asymmetry,  however,  is  not  a  kind  limited  to  the  aromatic 


THE  BENZENE   SERIES. 


95 


series,  V>«tjg  simply  pi  sppmal  fi^ae  o^jfrhe  geometrical  isom- 
erism  observed  in  the  unsaturate^^^esijforjit<w^ill  be  recalled 
that  if,  in  nialeic  and  fumaric  acids,  one  carboxyl  be  replaced 
by  hydrogen,  all  possibility  of  isomerism  is  at  once  lost. 

Similarly,  isomerism  has  been  observed  in  the  tetra-  and 
di-hydrophthalic  acids,  an  isomerism  explained  by  the  follow- 
ing formulas :  — 


H          COOH 
\   / 
C 


CH2 
I 

CH2 
/ 

\ 


H          COOH 
\  / 
C 


HC 

II 

HC(5> 
\ 


HC 

II 

HC(5) 
\ 


CH2 

I 
CH2 


\ 


H         COOH  COOH          H 

A6  ct'stetrahydroterephthalic  acid,    and  A6  frans-tetrahydroterephthalic  acid. 


H 


\ 


C 


HC 
II 

HC<5> 
\ 


COOH 

(2)CH 
II 
CH 


H         COOH 
\  / 

C 

/d)\ 

HC      (2)CH 


CH 


\ 


C 


\ 


\ 


H 


COOH 


COOH         H 


A2, 6  cj'sdihydroterephthalic  acid,      and     A2,  5  Zrans-dihydroterephthalic  acid. 

The  number  of  possible  isomeric  forms  of  the  hydro  acids 
is  quite  large,  since  they  are  capable  of  geometrical  isomerism, 
as  shown  above,  and  also  any  different  arrangements  of  the 
double  bonds  gives  a  new  isomeric  form.  For  example,  beside 


THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 


COOH 

i 

i 
C 

,  11  K 

^'-'\ 

HC(6) 

HC 

CH2 

1 

CH2 
f 

C 
1 
COOH 

the  two  dihydroterephthalic  acids  whose  formulas  are  given 
above,  we  may  have :  — 

H         COOH 
\   / 

C 

/   \ 
HC         CH2 

II  I 

HC         CH 

\  //      * 
C 
I 
COOH 

A4,  6  dihydroterephthalic  acid,    and    A8, 5  dihydroterephthalic  acid. 

It  thus  becomes  possible,  theoretically,  to  have  fifteen  hy- 
drophthalic  acids,  and  of  these l  eleven  are  known  and  their 
constitution  quite  well  determined.  It  has  been  von  Baeyer's 
great  work  to  prepare,  purify,  and  determine  the  constitution 
and  properties  of  these  different  isomeric  bodies,  and  to  de- 
velop the  subject  of  geometrical  isomerism  as  found  in  ring 
formations.  This  isomerism  is  not  limited  to  the  benzene 
series,  but  is  capable  of  a  more  general  application.  Accord- 
ding  to  von  Baeyer,  all  rings  have  the  carbon  atoms  in  a 
plane,  and  the  connected  radicals  may  lie  on  the  same  or  on 
opposite  sides  of  that  plane.  The  simplest  ring  known  is 
cyclopropane,"  x 

CH,          YV 


which  shows  the  isomerisni  which  might   be  expected   from 
von  Baeyer's  views. 

1  Lieb.  Ann.  d.  Chem.  269,  145. 


THE  BENZENE  SERIES.  97 

Another  illustration  of  geometrical  isomerism  in  benzene 
derivatives  may  be  found  in  benzene1  hexachlori.de.  Two 
isomeric  hexachlorides  have  been  found,  differing  in  physical 
properties  and  in  stability,  the  /?  variety  being  much  more 
stable  than  the  other.  Friedel  discusses  these  bodies  by 
applying  van't  Hoff's  hypothesis  to  Kekule's  plane  formula, 
and  pictures  the  benzene  molecule  as  made  up  of  three  pairs 
of  tetrahedrons,  the  pairs  being  joined  together  by  a  common 
apex,  and  the  individuals  of  each  pair  being  joined  together 
by  a  common  edge,  giving  the  following  figure  :  — 


Upon  formation  of  the  hexachloride,  the  double  bonds, 
represented  by  edges,  are  broken,  and  there  may  be  formed  a 
molecule,  in  which  the  chlorine  atoms  are  all  on  the  same  side 
of  the  plane  of  the  carbons,  giving  the  cis  isomer ;  or  four  of 
the  chlorine  atoms  may  be  on  one  side  and  two  on  the  other, 
giving  the  trans  isomer,  as  pictured  below. 

Friedel  judges  that  the  cis  molecule  is  the  ft  or  more  stable 
modification,  thinking  that  the  greater  symmetry  of  this  mole- 
cule may  account  for  the  comparative  stability,  greater  den- 

i  J.  Chem.  Soc.  60,  165;  Bull.  Soc.  Chem.  (3)  6,  130. 

7 


98 


THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 


sity,  and  higher  boiling-point  of  the  /?  hexachloride.  Friedel 
also  finds  a  connection  between  the  symmetry  of  the  mole- 
cular formula  and  that  exhibited  by  the  crystallized  substance. 
Thus  the  a  hexachloride  crystallizes  in  the  monoclinic  system, 
and  a  study  of  the  formula  suggested  by  Friedel  shows  that 
the  trans  molecule  has  one  and  only  one  plane  of  symmetry. 
The  £  variety  has  been  described  as  crystallizing  in  regular 
octahedrons,  but  Friedel  finds  that  these  crystals  affect  polar- 
ized light,  and  upon  careful  study  decides  that  these  octa- 
hedrons are  formed  by  grouping  together  eight  triangular 
pyramids  belonging  to  the  hexagonal  system,  and  the  figure 
shows  that  the  molecule  of  the  trans  hexachloride  possesses 
hexagonal  symmetry :  — 


Cis  hexachloride  of  benzene. 


Trans  hexachloride  of  benzene. 


Friedel's  discussion  is  very  interesting ;  but  it  has  already 
been  pointed  out  that  any  stereo-arrangement  of  the  benzene 
nucleus  must  at  present  be  accepted  with  caution.  Kekule's 
formula,  however,  appears  from  these  discussions  to  be  well 
adapted  to  being  brought  into  harmony  with  the  tetrahedral 
symbolism  used  in  the  other  series  of  organic  compounds. 


THE  BENZENE  SERIES.  99 

Another  formula  for  the  benzene  ring  has  been  devised  by 
Sachse,1  who  has  also  adopted  the  tetrahedron  as  the  geomet- 
rical figure  for  each  carbon  atom.  He  states  that  if  the  car- 
bon atoms  in  the  formulas  of  Kekule,  Ladenburg,  or  Glaus  be 
replaced  by  tetrahedrons,  figures  result  which  are  not  in  com- 
plete accordance  with  the  facts  observed,  and  particularly  in 
that  they  offer  no  explanation  of  the  fact  that  six  tetrahedrons 
form  the  most  stable  possible  combination,  and  that  in  the 
union  of  two  benzene  rings  in  their  most  stable  form,  as  in 
naphthalin  and  anthracene,  there  are  two  carbon  atoms  common 
to  the  two  rings.  These  facts  he  thinks  are  explained  per- 
fectly by  his  own  geometrical  mode  of  representation  ;  and  as 
there  are  some  very  interesting  points  about  his  formula,  it 
may  be  worth  while  to  consider  it  briefly  here. 

His  geometrical  formula  is  derived  by  taking  away  any  two 
parallel  faces  from  the  regular  octahedron,  and  placing  upon 
each  remaining  face  a  tetrahedron  whose  face  equals  the  face 
of  the  octahedron.  The  resulting  figure  has  then  six  free, 
solid  angles,  one  from  each  tetrahedron,  representing  the 
points  of  attachment  for  the  six  hydrogen  atoms.  An  exami- 
nation of  the  figure  shows  that  three  valencies  from  each  car- 
bon atom  are  so  taken  up  by  the  neighboring  carbons  that  a 
normal  union  of  another  element  through  these  affinities  can- 
not take  place  without  changing  the  entire  system.  This  six- 
membered  ring  is  then  the  most  stable  of  all  ring  formations, 
because  each  atom  is  so  combined  with  its  neighbor  that  it 
is  in  stable  equilibrium,  and  any  motion  in  the  molecule  must 
be  a  motion  of  the  molecule  as  a  whole,  any  periodic  oscilla- 
tion of  the  separate  atoms  being  impossible.  This  is  proved 
geometrically  as  follows  :  — 

i  Ber.  21, 2530. 


100        THE  DEVELOPMENT    OF    STEREO-CHEMISTRY. 


If  A  and  B  represent  the  projection  on  the  plane  of  the 
paper  of  two  tetrahedrons  joined  by  double  bonds,  according 
to  the  common  mode  of  representation  following  van't  HofFs 
hypothesis,  then  to  be  in  equilibrium,  ft  must  equal  /3',  and 


in  which  a  equals  the  regular  tetrahedral  angle,  70°  32',  /. 
/?  =  109°  28',  which  is  the  interfacial  angle  of  the  regular 
octahedron.  Now,  since  the  inside  of  our  molecule  is  an  octa- 
hedron, it  follows  that  fi  =  fif  for  each  pair  of  tetrahedrons,  and 
that  they  are  all,  therefore,  in  a  state  of  stable  equilibrium. 

Again,  since  /?  =  /?',  one  of  these  angles  being  opened  in- 
ward toward  the  centre  of  the  octahedron,  and  the  other  out- 
ward, it  becomes  possible  for  this  exterior  angle  to  become 
the  interior  angle  of  another  ring  exactly  like  the  first,  and 
these  two  rings  will  have  two  carbon  atoms  in  common,  and 
this  would  seem  to  be  the  most  stable  possible  way  of  com- 
bining two  rings.  This,  then,  explains  the  constitution  of 
such  bodies  as  anthracene  and  naphthalin,  for  only  in  this  way, 
keeping  two  carbon  atoms  in  common,  is  it  possible  for  both 
rings  to  retain  at  the  same  time  the  form  of  the  most  stable 
of  all  systems. 

The  main  peculiarity  of  this  conception  of  the  benzene  ring 
is  that  we  have  here  only  double  linkage,  each  atom  being 
doubly  bound  to  two  other  carbon  atoms,  —  a  state  of  affairs 


THE  BENZENE  SERIES.  101 

which  would  generally  be  considered  impossible,  since  it  would 
leave  no  place  for  the  hydrogen  atoms.  To  account  for  this, 
the  author  introduces  a  new  conception  of  the  union  of  carbon 
atoms  with  each  other,  peculiar  to  benzene  and  other  rings. 
He  assumes  that  when  two  carbon  atoms  are  combined  by 
double  linkage,  there  is  not  a  complete  neutralization  because 
the  valencies  meet  at  an  angle.  Hence  a  third  valency  from 
a  third  carbon  atom  is  used  in  completing  the  mutual  neutrali- 
zation. Therefore,  whenever  three  carbon  atoms  meet  in  one 
point  on  the  model,  there  is  this  distribution  of  affinities 
which  is  peculiar  to  ring  formations,  and  particularly  to 
benzene. 

Sachse l  has  also  offered  a  formula  for  hexamethylene,  which 
can  easily  be  derived  from  his  formula  for  benzene,  and  which 
shows  the  relation  between  the  two  bodies. 

Sachse's  geometrical  formula  for  benzene  possesses  the 
advantages  claimed  for  it  in  explaining  the  facts  mentioned. 
It  also  shows  a  possibility  of  two  kinds  of  double  linkage,  the 
ordinary  loose  kind  in  which  the  saturation  is  incomplete,  and 
this  more  stable  form  in  which  complete  saturation  is  effected 
by  the  presence  and  action  of  a  third  carbon  atom.  In  this 
conception,  the  six  atoms  forming  the  benzene  nucleus  are  not 
in  a  plane,  as  they  are  in  von  Baeyer's  theory,  and  in  some 
ways  this  commends  itself.  The  fact  that  the  hexa  com- 
pounds are  generally  more  stable  and  more  easily  formed  than 
the  penta  is  certainly  an  argument  against  von  Baeyer's 
theory,  and  that  theory  offers  no  explanation  of  the  stability 
of  the  double  bonds  which  must  be  present.  On  the  other 
hand,  in  the  present  state  of  our  knowledge  regarding  the 
nature  of  atoms  and  valence,  it  is  as  difficult  to  grasp  the  idea 
that  the  bonds  of  affinity  do  not  completely  saturate  each 

i  Ber.  23,  1363. 


102        THE  DEVELOPMENT    OF   STEREO-CHEMISTRY. 

other  because  they  meet  at  an  angle,  as  it  is  to  comprehend 
von  Baeyer's  "  strain  "  theory. 

It  has  also  been  urged  against  Sachse's  formula  that,  on  the 
same  principle,  a  number  of  other  bodies  ought  to  be  capable 
of  existence  which  have  never  been  discovered ;  for  example, 
such  a  body  as  that  represented  by  the  following  formula  : 

CH-CH 
\Y/ 
CH 

The  non-existence  or  non-discovery  of  certain  particular 
bodies  is,  however,  a  weak  argument  against  any  hypothesis, 
and  Sachse's  three-dimension  formula  for  benzene  must  be 
considered  an  interesting  and  ingenious  theory,  even  though 
the  time  may  not  seem  quite  ripe  for  adopting  any  such 
formula  at  present. 

Another  geometrical  formula  which  has  been  suggested  for 
the  benzene  nucleus  is  that  offered  by  Herrmann,1  and  this 
seems  to  have  its  origin  entirely  in  theoretical  speculations. 
For  hexahydrobenzene,  he  adopts  the  figure  of  the  regular 
hexahedron,  having  the  six  carbon  atoms  in  the  centre  of  the 
faces,  and  the  twelve  hydrogen  atoms  in  the  middle  points  of 
the  edges.  These  hydrogen  atoms  are  divided  into  two  sets, 
and  one  set  is  taken  away  in  the  formation  of  benzene.  This 
leaves,  as  the  geometrical  form  for  benzene,  the  figure  of  an 
octahedron  inscribed  in  a  cube,  the  six  carbon  atoms  occupy- 
ing the  angles  of  the  octahedron,  and  the  six  hydrogen  atoms 
the  centres  of  six  of  the  edges  of  the  cube,  arranged  in  such  a 
way  that  if  one  edge  contains  a  hydrogen  atom,  the  edge 
opposite  contains  none. 

The  plane  of  the  hydrogen  atoms  is  not  a  plane  of  sym- 
metry, but  by  projection  of  the  places  of  the  carbon  atoms 

i  Ber.  21, 1949. 


THE  BENZENE  SERIES.  103 

upon  this  plane,  the  following  figure  is  obtained,  in  which  the 
Roman  numerals  represent  the  carbon  atoms  :  — 


I,  II,  and  III  lie  over,  and  I',  II',  and  III'  under,  the  plane 
of  representation,  and  the  configuration  possesses  six  planes  of 
symmetry.  Adopting  this  formula,  if  two  hydrogen  atoms 
are  replaced  by  two  different  radicals,  we  may  get  two  con- 
figurations bearing  to  each  other  the  relation  of  object  and 
reflected  image,  as  is  shown  by  allowing  1  and  2'  to  exchange 
places  in  the  above  figure.  Analogy  would  lead  us  to  expect 
optical  activity  in  such  a  case,  but  no  such  activity  has  been 
observed  in  any  normal  benzene  derivative.  Herrmann  meets 
this  criticism  upon  his  formula  by  supposing  that  the  plane 
of  the  hydrogen  atoms,  though  not  a  geometrical,  is  an  optical 
plane  of  symmetry,  and  that  the  rotation  which  the  plane  of 
polarization  of  the  light  ray  entering  the  molecule  suffers 
through  the  influence  of  one  system  of  atoms  is  removed  by 
the  action  of  the  equal  and  oppositely  placed  system  on  the 
other  side  of  the  plane. 

The  speculations  of  Herrmann,  it  will  be  observed,  are  based 
entirely  on  geometrical  reasoning,  and  he  makes  no  assump- 
tions whatever  in  regard  to  the  nature  or  valence  of  the  atoms, 
being  occupied  entirely  with  their  geometrical  distribution. 


104        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

Such  formulas  as  these  are  interesting  principally  as  show- 
ing the  kind  of  work  which  is  being  done,  and  the  speculations 
which  are  being  offered  in  stereo-chemistry.  Perhaps  they 
show  also,  that  there  is  some  danger  of  pushing  stereo-chemi- 
cal ideas  too  far  and  too  rapidly.  So  many  of  these  ideas 
have  their  basis  in  well-grounded  facts  that  it  would  be  a  pity 
to  cast  valuable  and  helpful  suggestions  and  symbols  into 
disrepute  by  a  rash  and  definite  picturing  of  molecules,  with- 
out sufficient  grounds.  While  all  of  these  geometrical  repre- 
sentations of  the  benzene  nucleus  are  interesting,  in  so  far  as 
they  help  to  explain  the  properties  of  bodies,  great  caution 
should  be  used  in  adopting  any  one  of  them. 


IV. 

THE   STEEEO-CHEMISTEY  OF  NITROGEN. 

IT  has  been  shown  in  the  foregoing  chapters  that  carbon 
compounds  are  capable  of  exhibiting  two  different  kinds  of 
isomerism,  which  agree  in  this  particular,  that  such  isomeric 
forms  possess  the  same  structural  constitution,  and  can  only 
be  explained  on  the  assumption  of  a  different  configuration 
of  the  atoms  in  space.  These  two  kinds  of  isomerism  have 
been  defined  as  physical  and  steie.Q-chemical. 

More  recently  there  has  been  an  effort  to  carry  over  the 
ideas  of  the  stereo-chemistry  of  carbon  to  other  elements, 
notably  nitrogen,  and  some  cases  of  isomerism  have  been 
observed  in  nitrogen  compounds  inexplicable  on  the  structural 
theory,  which  seem  to  justify  this  attempt.  It  is  noticeable, 
however,  that  such  cases  of  isomerism  have  always  been  ob- 
served in  organic  compounds  containing  nitrogen. 

The  nitrogen  atom  differs  essentially  from  the  carbon,  in 
that  it  has  a  varying  valence.  The  vast  preponderance  of  the 
compounds,  which  can  be  explained  on  the  supposition  that 
carbon  has  a  valence  of  IV,  will  perhaps  justify  this  state- 
ment, though  neither  van't  Hoff's  nor  any  other  theory  offers 
a  thoroughly  satisfactory  explanation  for  such  a  body  as  car- 
bon monoxide.  But  nitrogen  differs  essentially  from  carbon, 
in  that  compounds  of  both  triad  and  pentad  nitrogen  are  of 
common  occurrence.  This  at  once  brings  before  us  several 


106         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


very  difficult  questions.  Are  these  two  kinds  of  valencies  of 
equal  or  unequal  value  ?  Do  they  all  act  in  the  same  plane, 
or  are  they  like  the  valencies  of  carbon  distributed  in  space, 
and  how  is  it  possible  for  us  to  represent  by  any  geometrical 
figure  an  atom  with  variable  valence  ?  These  are  questions, 
none  of  which,  perhaps,  can  be  answered  to  our  entire  satis- 
faction in  the  present  state  of  our  knowledge,  but  they  are 
very  suggestive  of  thought. 

Whatever  may  be  said  in  answer  to  the  question  of  the 
equality  or  non-equality  of  the  five  nitrogen  bonds,  it  must  be 
conceded  that  they  differ  in  one  essential,  three  are  neces- 
sarily active  in  all  compounds,  the  other  two  may  be  dormant. 
If  we  endeavor  to  get  a  three-dimension  formula  for  the  nitro- 
gen atom,  it  would  seem  to  be  necessary  to  suppose  that  the 
valencies  are  not  all  equal,  since  it  is  impossible  to  find  a  sym- 
metrical arrangement  of  five  points  in  three-dimension  space. 

Although  carbon  itself  differs  widely  from  nitrogen,  the 
CH  group  seems  to  be  replaceable  by  nitrogen  in  a  number  of 
compounds ;  for  example,  acetylene,  CH  ==  CH,  corresponding 
to  hydrocyanic  acid,  CH  ==N,  and  pyridin,  C6H5K,  to  benzene, 
C5H5CH.  This  has  given  rise  to  the  suggestion  that  such 
corresponding  molecules  may  have  corresponding  configura- 
tions, as  shown  by  the  following  figures :  — 


THE   STEREO-CHEMISTRY  OF  NITROGEN. 


107 


In  such  a  conception  as  this,  the  triad  nitrogen  is  pictured 
as  having  its  valencies  in  three  of  the  angles  of  a.  tetrahedron, 
whose  fourth  angle  is  Qccjipiad  by  the  nitrogen  itself.  In  this 
case  it  is  evident  that  the  three  valencies  do  not  lie.  in  the 
same  plane  ;  and  a  careful  study  of  many  nitrogen  compounds 
has  led  some  investigators  to  conclude  that,  though  in  certain 
compounds  the  three  valencies  all  work  in  the  same  plane,  in 
others  they  are  bent  from  that  position. 

Van't  Hoff1  was  the  first  to  suggest  a  three-dimensional 
arrangement  of  the  radicals  attached  to  the  nitrogen  atom. 
According  to  him,  the  radicals  attached  by  the  three  valencies 
which  are  always  active  must  lie  at  equal  distances  from  the 
nitrogen  atom.  The  other  two  valencies  differ  from  these  in 
that  they  may  be  dormant,  and  from  each  other  in  that  one 
holds  preferably  negative,  the  other  positive,  atoms  or  radicals. 
In  any  geometrical  figure  to  represent  these  facts,  indicating 
the  first  set  of  valencies  by  1,  2,  and  3,  and  the  second  by  4 
and  5,  each  of  the  latter  should  be  equidistant  from  1,  2,  and 
3.  The  form  suggested  by  van't  Hoff  is  a  cube,  in  which  the 
nitrogen  occupies  the  centre,  and  which  is  represented  by  the 
accompanying  figure. 


1 

I  ® 

JL 

a 

^ 

^ 

Lossen  has  discovered  certain  hydroxylamine  derivatives, 
which,  though  they  may  be  explained  on  the  ground  of  struc- 
tural differences  without  any  serious  inconvenience,  seem  to 


1  Ansichten  ueber  organische  Chemie,  p.  79. 


108        THE  DEVELOPMENT  OF    STEREO-CHEMISTRY. 

van't  Hoff  to  be  explained  more  naturally  on  the  assumption 
that  there  are  three  different  kinds  of  hydrogen  atoms  in 
hydroxylamine.  He  would,  then,  write  the  formula  for  this 
latter  body,  NO .  Hs,  and,  using  the  geometrical  formula  given 
above,  the  oxygen  may  be  assumed  to  occupy  places  1  and  2, 
whereby  there  are  left  for  the  three  hydrogen  atoms  only 
places  3,  4,  and  5,  no  two  of  which  are  alike  in  their  relations 
to  the  other  atoms. 

Similar  attempts  to  determine  a  three-dimensional  formula 
for  the  nitrogen  atom  have  been  made  by  Willgerodt,1  and  by 
Burch  and  Marsh,2  who  were  apparently  ignorant  of  van't 
Hoff's  work  in  this  direction,  and  who  pictured  the  nitrogen 
as  a  double  tetrahedron. 


They  attempt  to  reconcile  the  varying  valence  of  nitrogen 
with  this  conception  by  supposing  that  two  atoms  of  nitrogen, 
in  their  apparently  triad  condition,  may  combine  with  one 
another  by  their  two  available  affinities  to  form  a  condensed 
molecule,  and  that  such  molecules  are  readily  dissociated  by 
heat  into  the  ordinary  molecules  containing  quasi  triad  nitro- 
gen. Experiments  with  ethylamine  vapor  seemed  to  show 
that,  to  some  extent  at  least,  this  vapor  breaks  up,  when 

1  J.  pr.  Chem.  [2],  37,  449;  41,  291,  526. 

2  J.  Chem.  Soc.  55,  656. 


THE   STEREO-CHEMISTRY   OF  NITROGEN.  109 

heated  from  17°  to  100°,  into  a  larger  number  of  molecules. 
According  to  this  view,  the  valence  of  the  nitrogen  is  essen- 
tially V,  and  when  found  with  apparently  a  valence  of  III, 
we  are  dealing  with  molecules  which  have  been  dissociated, 
and  in  which  the  two  available  bonds  are  too  feeble  at  the 
temperature  of  observation  to  enable  the  two  molecules  to 
hold  together. 

They  assume,  also,  that  different  elements  have  different 
effects  in  weakening  or  strengthening  these  two  bonds.  Thus, 
that  oxygen  strengthens  and  hydrogen  weakens  these  bonds 
is  inferred  from  the  fact  that  NH8  =  NH3  and  NH4  —  NH4  are 
non-existent,  whereas  N02  — N02  is  well  known.  They  sug- 
gest that  the  union  of  an  element  of  strong  affinity  with  one 
of  the  valencies  might  render  the  other  four  equal  toward 
atoms  of  weaker  affinities  ;  for  example,  representing  Nv  by  the 
double  tetrahedron  as  above,  if  one  angle  is  taken  up  by 
chlorine,  the  remaining  four  are  symmetrical  with  reference 
to  one  point  in  the  molecule,  and  may  be  satisfied  by  four 
equal  atoms,  as  in  NH4C1.  These  speculations  are  ingenious 
and  interesting,  especially  in  their  explanation  of  varying 
valence,  but  seem  hardly  sufficient  to  justify  us  in  adopting 
any  three-dimensional  formula  for  the  nitrogen  atom  at  present. 

In  studying  the  cases  of  nitrogen  compounds  in  which  either 
of  the  kinds  of  geometrical  isomerisrn  observed  in  carbon 
compounds  is  conceivable  or  has  been  found,  it  may  be  well  to 
consider  them  under  the  following  heads  :  — 

I.  Compounds  in  which  nitrogen  exhibits  single  linking. 

(a)  Xtiad-sitrogen. 

(b)  Peatad  nitrogen. 

II.  Compounds  in  which  nitrogen  shows  double  linking. 

(a)  Doubly  linked  to  carbon. 

(b)  Doubly  linked  to  nitrogen. 


110        THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

Under  I.  (a),  in  case  the  nitrogen  is  combined  with  three 
different  radicals,  giving  the  general  formula, 


it  is  conceivable,  as  has  been  before  shown,  that  the  nitrogen 
should  take  the  place  of  CH  in  a  tetrahedron,  giving  an 
arrangement  capable  of  existing  in  two  forms,  one  of  which 
should  be  the  reflected  image  of  the  other.  This  should  give 
rise  to  a  physical  isomerism,  manifested,  as  in  the  case  of  com- 
pounds containing  asymmetric  carbon,  by  optical  activity. 
All  efforts,  however,  to  discover  optical  activity  in  bodies  of 
this  general  formula  have  proved  unavailing,  therefore  it  is 
generally  conceded  that  the  three  valencies  in  bodies  of  this 
type  lie  in  a  plane,  making  geometrical  isomerism  impossible. 

I.  (b)  Compounds  containing  pentad  nitrogen  with  single 
linking. 

No  optical  activity  has  been  observed  in  compounds  of  this 
class  except  in  cases  where  no  two  of  the  radicals  combined 
with  the  nitrogen  are  identical  ;  that  is,  in  bodies  of  the  gen- 
eral type, 


e  d 

Le  Bel  has  succeeded  in  obtaining  methyl-ethyl-propyl-isobutyl- 
ammonium  iodide  in  active  form,1  and  from  this  also  other 
salts  with  the  same  power,  but  with  great  instability.  Very 
few  such  cases  are  known,  but  they  are  sufficient  to  indicate 
that,  in  the  completely  ^asymmetric  nitrogen  molecule,  there 

1  Compt.  rend.  112,  724. 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  Ill 

is  the  capability  of  the  same  kind  of  isomerism  as  that 
exhibited  in  a  molecule  containing  asymmetric  carbon.  It 
has  been  suggested  that,  in  these  cases,  the  (NxJ)IV  being 
equivalent  to  C17,  the  four  valencies  of  this  group  may  be  dis- 
tributed in  space  in  the  same  way  as  in  the  carbon  atom,  and 
therefore  there  may  be  a  possibility  of  the  same  kind  of 
isomerism. 

These  are  probably  the  only  compounds  yet  known  in  which 
optical  activity  is  not  connected  with  the  presence  of  asym- 
metric carbon,  but  the  existence  of  these  compounds  is  in 
complete  accordance  with  Le  Bel's  statement  of  the  funda- 
mental principle  of  stereo-chemistry  :  "  If  a  substance  is 
derived  from  the  primitive  type  MA4,  by  the  substitution  of 
three  different  atoms  or  radicals  for  A8,  its  molecule  will  be 
asymmetric  and  will  possess  the,  jpowej^  of  rotation."  In  this, 
M  stands  for  any  radical  simple  or  complex,  so  there  is  noth- 
ing in  this  form  of  statement  which  would  limit  the  power  of 
causing  rotation  to  a  single  carbon  atom,  to  which  four  differ- 
ent radicals  are  attached.  As  a  matter  of  fact,  however,  the 
phenomenon  is  almost  universally  met  with  in  these  latter 
compounds,  so  that  the  two  forms  of  stating  this  fundamental 
principle  of  stereo-chemistry,  van't  Hoff's  and  Le  Bel's,  have 
frequently  been  considered  identical. 

In  addition  to  these  optically  active  nitrogen  compounds, 
Le  Bel  has  observed  dimorphism  in  trimethyl-isobutyl- 
ammonium  chloride,1  which  he  attributes  to  stereo-isomerism, 
and  the  same  phenomenon  has  been  observed  by  Schryver  and 
Collie  in  methyl-ethyl-pentyl-ammonium  chloride.2  Tri-ethyl- 
benzyl-ammonium  iodide  is  another  body  which  is  apparently 
capable  of  existing  in  different  isomeric  forms,  according  to 
the  order  in  which  the  hydrocarbon  radicals  are  combined 

1  Compt.  rend.  110, 144.  2  Chem.  News.  63, 174. 


112        THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 

with  the  nitrogen,1  and  of  this  van't  Hoff  says,  "  These  iso- 
mers  may  certainly  be  due  to  a  difference  in  the  value  of  the 
nitrogen  affinities,  but  may  equally  be  due,  and  this  is,  per- 
haps, the  more  natural  assumption,  to  a  different  arrangement 
in  space  of  the  groups  combined  with  the  nitrogen." 

II.  (a)  Compounds  containing  nitrogen  joined  by  double 
bonds  to  a  carbon  atom. 

This  is  by  far  the  most  important  class  of  nitrogen  com- 
pounds in  which  geometrical  isomerism  has  been  observed. 
ID  bodies  of  the  type, 

\  X 

C 

II 

N 
,  \ 

an  isomerism  has  been  quite  commonly  detected  which  is  very 
closely  related  to  the'  stereo-chemical  isomerism  observed  in 
doubly  linked  carbon  compounds.  Such  geometrical  isomers 
differ  in  all  their  physical  properties,  and  in  those  chemical 
properties  which  depend  on  inter-molecular  reactions,  the 
different  distance  of  the  atoms  from  each  other,  etc, 
'  This  kind  of  isomerism  is  almost  entirely  limited  to  oximes 
rand  their  derivatives,  but  has  also  beeiu observed  in  some 
I  ethers  of  hydrp^mjfi  jp"if|g/ani1  in  nnmr  hydrazones.  The 
principal  oximes  studied  are  benzil  monoxime, 

/  1  NOH 

JC6H5-C-CO-C«] 

benzil  dioxime, 

-    ^  TT          r\  r\ 

^g£lr   —    \J       **  "      \J   •"" ' 
II  II 

NOH  NOH 

Ber.  X.,  45T8uy,  Dtu,  ^J&f  1152,  and  1634. 


THE   STEREO-CHEMISTRY   OF   NITROGEN.  113 

and  benzaldoxime, 

H 


Two  different  benzil  dioximes  were  first  discovered  by 
Victor  Meyer  and  Goldschmidt  ;  and  the  structural  identity  of 
these  two  bodies  was  proved  by  Y.  Meyer  and  Auwers,1  who 
later  also  discovered  a  third  isomeric  form  with  same  struc- 
ture as  the  other  two.2  The  two  isomeric  benzil  monoximes 
have  also  been  carefully  studied,  and  proved  to  be  structu- 
rally identical  by  Auwers  and  Meyer,8  and  by  Auwers  and 
Dittreich.4 

The  principal  properties  of  the  two  benzil  monoximes  which 
are  of  importance  in  this  discussion  are  as  follows  :  They 
are  both  formed  from  benzil,  C6H6  —  CO  —  CO  —  C6H5,  and 
hydroxylamine  at  the  ordinary  temperature.  Both  are  decom- 
posed by  hydrochloric  acid  into  benzil  and  hydroxylamine. 
They  combine  further,  even  at  ordinary  temperatures,  with 
hydroxylamine,  and  produce  two  different  dioximes.  Both 
combine  with  phenyl  hydrazin  to  form  hydrazones,  and  the 
presence  of  carbonyl  in  both  is  established  beyond  doubt. 
They  are  both  optically  inactive.  To  each  belongs  the  struc- 
tural formula 

C6H5          NOH 
\  // 

C 

I 

C 

'  ^ 
C6H6         0 

Here  we  have,  then,  a  case  of  isomerism  clearly  similar  to 
the  stereo-chemical  isomerism  of  organic  bodies,  and  yet  with 

1  Ber.  21,  784.        *  Ber.  22,  705.        *  Ber.  22,  537.        *  Ber.  22,  1996. 

8 


114        THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 

no  double  linking  between  the  carbon  atoms.  According  to 
the  theories  before  given,  there  should  be  no  possibility  of 
isomerism  in  a  body  of  this  type  unless  due  to  structural 
differences.  Experiments  done  with  other  bodies  containing 
an  NOH  group  had  shown  that  there  were  two  possible  ar- 
rangements inside  this  group, 

//  //O 

N-OH        and        R£ 

NH 

Experiments  by  which  the  latter  group  had  been  proved  pres- 
ent in  some  compounds  were  carefully  repeated  by  Auwers 
and  Dittreich,  who  proved  conclusively  that  in  this  case  both 
isomers  contained  the  same  group, 

OH 


In  the  dioximes  the  conditions  are  very  similar  to  those  in 
the  monoximes,  only  that  we  have  here  three  different  bodies, 
all  of  which  must  be  conceded,  after  the  careful  investigation 
of  Auwers  and  Meyer,  to  be  represented  by  the  same  struc- 
tural formula  :  — 

C6H,         NOH 


C6H5         NOH 

(1) 

This  isomerism,  then,  is  not  to  be  explained  structurally, 
nor  by  space  configurations  according  to  van't  Hoff's  theories, 
for  if  we  should  write  the  formula  as  follows, 


THE   STEREO-CHEMISTRY  OF  NITROGEN.  115 

C6H5          NOH 

V 

I 
c 

//  \ 

NOH         C6H5 

(2) 

(2)  could  be  derived  from  (1)  by  simple  rotation  of  a  carbon 
atom,  and  should,  therefore,  according  to  van't  Hoff,  represent 
the  same  isomeric  form. 

In   explanation   of    this   remarkable  phenomenon,   various 
theories  have  been  advanced. 

I.  Theory  that  this  isomerism  may  be  due  to  the  inequality 
of  the  nitrogen  bonds.     This  hypothesis  has  hardly  been  suffi- 
ciently  developed  to   be   dignified  by   the   name   of   theory. 
Van't  Hoff,  without  discussing  the   subject  of  the  nitrogen 
compounds  in  detail,  merely  offers  this  suggestion  as  a  pos- 
sible explanation.     He  says :  "  We  do  not  know  the   nature 
of  the  union  of  the  carbon  atoms  with  the  NOH  groups,  but  it 
is  quite  possible  that  the  two  bonds  which  effect  the  union 
are  of  unequal  value.     Hence  the  two  carbon  atoms  will  be 
asymmetric,  and  without  being  deprived  of  free  rotation,  will 
give  rise  to  two  isomers,  inactive  and  analogous  to  racemic 
and  mesotartaric  acids." 

II.  Theory  of  limited  rotation.     This  is  the  theory  which 
was  developed  by  Auwers  and  Meyer.     These  two  eminent 
chemists  differed  in  some  details  as  to  the  ultimate  cause  of 
the  phenomenon,  but  agreed  in  assuming  that  in  some  cases 
there  might  be  two  or   more   different  stable  arrangements 
around  singly  linked  carbon  atoms. 

In  1888,  the  two  then-known  benzil  dioxinies  were  carefully 
studied  by  Auwers  and  Meyer,  who  showed  that  they  were 


116        THE   DEVELOPMENT   OF    STEREO-CHEMISTRY. 

structurally  identical,  and  that  no  theory  had  ever  been 
brought  forward  which  would  account  for  such  isomeric  bodies. 
The  explanation  which  they  proceeded  to  offer  was  a  modifica- 
tion of  van't  Hoff's  second  hypothesis.  While  they  acknowl- 
edged that  the  principle  of  free  rotation  was  to  be  accepted 
for  the  vast  majority  of  singly  linked  carbon  compounds,  they 
assumed  that  there  might  be  exceptions  to  this  rule.  For  the 
benzil  dioximes,  accepting  van't  Hoff's  hypothesis,  there 
should  be  only  one  configuration  possible, 

NOH          C6H5 


CH 


NOH 


On  the  other  hand,  assuming  that  the  different  configurations 
produced  by  the  rotation  of  one  of  the  carbon  atoms  represent 
different  isomeric  modifications,  there  would  be  a  possibility 
of  three  different  forms  shown  by  the  following  formulas,  in 
which  the  group  NOH  is  represented  by  n  n  :  — 


C6H5 


n 

n 

\  / 

C 

I 

C 

C6H5    I    n 

n 

(1) 


n 


C6H5 


\ 


C6H5 


n 


n 


n 


n 

(2) 


\ 


C6H5 

(3) 


At  the  time  when  these  speculations  were  offered,  only  two 
of  these  dioximes  were  known,  but  in  the  following  year,  a 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  117 

third  isomer  was  discovered  by  Auwers  and  Meyer,  who  felt 
that  their  theory  received  great  support  from  this  discovery. 
This  third  isomer,  the  y  benzil  dioxime,  is  characterized  by 
its  instability,  slight  heat  under  all  circumstances  changing  it 
to  the  ft  form,  and  by  its  tendency  to  form  an  anhydride, 
therefore  formula  (1)  was  assigned  to  it  ;  but  it  would  be 
impossible  to  decide  which  of  the  other  formulas  belonged  to 
each  of  the  other  two  forms.  Hantzsch  argued  that  (2)  and 
(3)  would  not  represent  the  comparative  properties  of  the  a 
and  ft  forms  satisfactorily.  It  might  be  expected  that  their 
properties  would  be  closely  similar,  and  that  they  would  differ 
only  as  right  and  left  tartaric  acids,  but  this  is  not  found  to 
be  the  case.  Leaving  other  properties  out  of  consideration, 
there  is  a  much  greater  difference  in  stability  than  might  be 
expected  from  these  formulas,  the  ft  variety  being  consider- 
ably more  stable  than  the  a. 

A  second  assumption  made  by  Meyer  and  Auwers  is  free 
from  this  objection,  and  is  yet  based  on  similar  ideas..  They 
assumed  that  in  the  system 


two  positions  of  stable  equilibrium  might  be  reached,  one 
when  the  combining  radicals  were  in  the  same  vertical  line, 
and  the  other  when  they  were  midway  between  these  posi- 
tions. Then  for 


configurations  are  shown  by  Auwers  to  be  possible,  repre- 
sented to  an  observer  looking  from  above  upon  the  molecule 
by  the  following  symbols  :  — 


118        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

&' 


(3)  and  (4)  represent  pairs  of  enantiomorphous  configurations, 
which  may  be  considered  identical.  On  this  assumption, 
there  were  shown  to  be  four  possible  configurations.  This 
was  open  to  the  objection  that  only  three  isomers  had  been 
discovered  j  and  though  this  was  only  negative  evidence,  their 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  119 

position  would  have  been  greatly  strengthened  if  a  fourth 
form  had  been  obtained. 

In  1890,  Robert  Behrend1  published  an  article  on  the  stereo- 
chemistry of  nitrogen  compounds,  in  which  he  refers  their 
configuration  to  electrical  polarity.  He  assumes,  for  example, 
that  ammonia  has  a  positive  and  a  negative  pole,  correspond- 
ing to  the  two  dormant  bonds.  If  now,  two  atoms  of  hydro- 
gen be  replaced  by  other  radicals,  their  position  will  be 
necessarily  fixed  by  their  electrical  character.  His  ideas  in 
regard  to  the  configuration  of  the  monoximes  and  dioximes 
agreed  with  those  of  Auwers  and  Meyer,  but  whereas,  hereto- 
fore, the  reason  for  the  grouping  of  the  system  had  been 
found  in  the  carbon  atoms,  he  found  it  in  the  directive  work- 
ing of  the  radicals  attached  to  the  carbon. 

Auwers,  in  his  later  ideas  as  set  forth  in  the  "  Entwicke- 
lung  der  Stereochemie,"  differed  somewhat  from  Meyer  as  to 
the  fundamental  cause  for  these  stable  configurations.  Meyer 
assumed  two  different  kinds  of  single  union  between  carbon 
atoms,  only  one  of  which,  by  far  the  most  common  kind, 
allows  unhindered  rotation.  He  arrived  at  this  conclusion 
from  some  very  interesting  speculations  made  by  himself  and 
Eiecke  2  concerning  the  nature  of  the  carbon  atom.  Auwers, 
on  the  other  hand,  thinks  that  two  singly  linked  carbon  atoms 
are  rotatable  under  all  circumstances ;  that  at  the  instant  of 
formation  of  a  molecule,  rotation  begins,  but  is  quickly 
checked  by  the  directive  affinities  of  the  atoms,  and  goes  over 
into  an  oscillatory  motion  around  a  position  of  equilibrium ; 
finally,  that  in  each  case  it  depends  on  the  chemical  nature  of 
the  combining  radicals  whether  only  one  or  several  positions 
of  equilibrium  can  have  a  lasting  existence  in  the  form  of 
isorneric  modifications.  Auwers  agrees  with  Behrend  in  con- 

i  Ber.  23,  454.  2  Ber.  21,  946. 


120        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

sidering  the  electrical  character  of  the  radicals  combined  with 
carbon  as  of  great  importance  in  determining  whether  only 
one  or  several  stable  positions  of  equilibrium  are  possible. 
In  case  the  radicals  are  all  of  nearly  the  same  electrical  char- 
acter, as  in  the  oximes,  he  thinks  several  such  positions  are 
possible,  whereas,  if  there  is  a  marked  difference  in  electrical 
character,  the  positive  and  negative  radicals  will  tend  to  draw 
as  near  together  as  possible  and  give  only  one  form.  As  an 
illustration  of  this,  he  points  to  dimethyl  glyoxime, 

CH8         NOH 

s  //. 

C 

I 
C 

//\ 

NOH         CH8 

which  exists  in  only  one  form,  according  to  his  theory  on 
account  of  the  fact  that  methyl  is  more  positive  than  benzil, 
and  therefore  has  a  stronger  attraction  for  the  NOH  group, 
while,  on  the  other  hand,  he  finds  that  many  derivatives  of 
benzil  behave,  in  regard  to  'oxime  formations,  exactly  like 
benzil  itself. 

These  theories,  it  will  be  observed,  all  explain  the  observed 
phenomenon  by  reference  to  the  carbon  rather  than  the  nitro- 
gen atoms,  and  demand  for  certain  cases  an  exception  to  van't 
HofPs  second  hypothesis,  which  states  that  all  configurations 
which  can  be  derived  from  each  other  by  simple  rotation  of 
one  carbon  atom,  must  be  considered  identical.  In  this  expla- 
nation of  the  isomerism  of  the  oximes,  no  special  stress  is  laid 
upon  the  nitrogen  atoms  except  that  attention  is  called  to  the 
fact  that  the  NOH  group  is  of  much  the  same  electrical  char- 
acter as  C«H5.  A  similar  isomerism  might  then  be  expected 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  121 

in  singly  linked  carbon  compounds  containing  no  nitrogen, 
but  radicals  of  nearly  the  same  electrical  character.  It  has 
already  been  mentioned  that  an  isomerism  has  been  observed 
in  the  benzil  carbonic  acids  and  a  very  few  other  compounds 
which  Meyer  would  explain,  as  in  the  case  of  the  oximes,  on 
the  supposition  of  limited  rotation.  Very  few  such  bodies  are, 
however,  known  with  certainty.  Undoubtedly,  if  careful 
study  in  the  future  should  result  in  the  knowledge  of  well- 
authenticated  cases  of  singly  linked  carbon  compounds  con- 
taining no  asymmetric  carbon  and  no  nitrogen,  and  yet 
exhibiting  an  isomerism  incapable  of  explanation  from  struc- 
tural differences,  it  would  be  a  strong  support  to  the  Auwers 
and  Meyer  theory. 

One  more  theory  remains  to  be  mentioned,  that  of  Hantzsch 
and  Werner,  which  was  advanced  in  1890,  and  which  refers 
the  isomerism  of  the  oximes  and  similar  bodies  containing 
nitrogen  doubly  bound  to  carbon,  to  the  stereo-arrangement  of 
the  groups  combined  with  the  nitrogen.1  They  assume  that 

bodies  of  the  type 

\  / 
C 
II 

N 
\ 

can  exhibit  a  geometrical  isomerism  entirely  analogous  to 
that  shown  in  bodies  of  the  >C=C<  type,  and  that  this 
isomerism  is  due  to  the  fact  that  in  these  compounds  the 
three  valencies  of  the  nitrogen  atom  do  not  all  lie  in  the  same 
plane,  one  being  drawn  out  of  the  plane  of  the  other  two  by 
the  attraction  of  other  atoms  or  radicals  in  the  molecule. 
According  to  this  view,  as  has  been  already  intimated,  the 
nitrogen  atom  may  be  pictured  as  lying  in  one  of  the  solid 
1  Ber.  23,  11,  and  Grundriss  der  Stereo-chemie. 


122        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


angles  of  a  tetrahedron,  and  exerting  its  affinities  in  the  direc- 
tion of  the  three  edges.  Then,  whenever  a  nitrogen  atom  is 
united  to  carbon  by  two  bonds,  a  possibility  of  isomerism 
would  exist,  as  shown  by  the  following  figures  :  — 


b 


and 


or, 


a         b 

\  / 

C 


d 


and 


a         b 

\    / 

C 


N 


\ 


According  to  this  view,  the  valency  attaching  d  to  ^Vis  turned 
from  the  plane  of  the  other  two  by  its  attraction  either  for  a 
or  b. 

They  would  then  explain  the  isomerism  observed  in  benzil 
monoxime  by  the  formulas  :  — 


\ 


CO-C6H6 


C 

II 
N 


and 


CO-C6H5 


OH 


OH 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  123 

and  for  the  benzil  dioximes  :  — 

C6H5-  C C  -  C6H5     C6H5  -  C C  -  C6H5 

II  II  II  II 

N-OH   HO-N  N-OH    N-OH 

(1)=7  (2)=« 

and 

C6H5-C C-C6H5 

HO-N  N-OH 

(3)  =jS 

For  the  three  latter  isomers,  he  assigns  the  formulas  as  indi- 
cated above,  giving  (1)  to  the  least  stable  form,  which  readily 
goes  over  into  its  anhydride,  and  assuming  that  (3)  would 
represent  the  configuration  most  favorable  for  stability. 

The  principal  objections  brought  by  Hantzsch  and  Werner 
against  the  theory  of  Auwers  and  Meyer  were  as  follows :  — 

1.  It  does  not  express  the  parallelism  between  the  stereo- 
isomeric  carbon  and  stereo-isomeric  nitrogen  compounds.     A 
number  of  compounds  are  known,  including  the  oximes  and 
their   derivatives,   which  contain  nitrogen,    doubly  linked  to 
carbon,  and  which  exhibit  an  isomerism  closely  similar  to  that 
shown  by  doubly  linked   carbon  compounds.     Hantzsch  and 
Werner  assign   to   such    bodies   formulas    which   show   this 
analogy,   whereas   Meyer  and  Auwers   throw   it   back  upon 
singly  linked  carbon  atoms,  and  the  parallelism  is  entirely 
lost. 

2.  The  theory  of  Auwers  and  Meyer  can  only  be  applied  to 
compounds  in  which  the  carbon  atom,  attached  by  two  bonds 
to  nitrogen,  is  also  connected  with  another  carbon  atom.     It 
therefore  leaves  out  of  account  several  bodies  whose  isomer- 
ism is  plainly  of  the  same  kind  as  that   which  their  theory 


124       THE  DEVELOPMENT  OF  STEKEO-CHEMISTRY. 

seeks  to  explain.     It  offers  no  explanation  for  the  isomerism 
of  the  hydrazones  ;   such  bodies,  for  example,  as 

CH30-C6H4  NH-C6H5 

\  / 


C6H6 

which  are  explained  by  Hantzsch  and  Werner  on  exactly  the 
same  principle  as  the  oxinies.  Also  two  different  ethyl 
hydroxamic  acids  have  been  discovered  by  Lossen,  unaccount- 
able on  the  structural  theory,  and  not  to  be  explained  by  the 
theory  of  Auwers  and  Meyer,  but  given  by  Hantzsch  and 
Werner  the  following  formulas  :  — 

C8H5  -  C  -  OC2H5  C6H5  -  C  -  OCaH5 

II  and  II 

N-OH  HO-N 

In  this  connection,  the  benzaldoximes  have  received  the 
greatest  attention.  If  these  two  bodies  can  be  proved  to  be 
structurally  identical,  it  would  be  almost  irrefutable  evidence 
against  the  Meyer  and  Auwers  theory,  since  they  are  so  closely 
related  to  the  other  oximes  that  any  theory  which  would 
account  for  one  on  space  relations  should  also  include  the 
other;  and  yet  their  formula,  C6H5  —  CH^NOH,  would  ex- 
clude the  application  of  the  theory  of  limited  rotation  of 
singly*  linked  carbon  atoms. 

The  iso-benzaldoxime  was  discovered  by  Beckmann1  in 
1889,  and  after  a  careful  study  of  its  reactions,  he  was  led  to 
give  it  the  formula  2 

1  Ber.  20,  2766. 

2  Ber.  22,  429,  514,  1531,  1588;  23,  1680. 


THE   STEREO-CHEMISTRY  OF  NITROGEN.  125 

C6H6  -  CH  -  NH 
\   / 
O 

The  reactions  which  led  him  to  the  conclusion  that  the  two 
benzaldoximes  were  structurally  different  were  the  follow- 
ing :— 

1.  The  benzyl  ethers  of  the  two  oximes,  by  heating  with 
hydrochloric  acid  in  presence  of  benzaldehyde,  give  two  differ- 
ent benzyl   hydroxylamines.     Behrend   and   Leuchs1  showed 
that  these  last  compounds   have   different   structures,  which 
correspond  to 

H  H 

\  / 

XN-O-C7H7        and      C7H7-N-OH 

H 

(«)  (0) 

2.  Conversely,  by  action  of  these  two  different  benzylhy- 
droxylamines  on  benzaldehyde,  he  found  that  he  could  syn- 
thetically produce  the  benzyl  ethers  of  the  two  benzaldoximes, 
reactions  which  he  expressed  as  follows :  — 

H 
/ 

N-H  +  C6H5  .  COH  =  H20+     N  =  CH  .  C6H6 

\  \ 

OC7Hr  OC7H7 

C7H7  CLHr 

/  / 

N-H  +  C6H5  .  COH  =  H20  +    N-CH.C6H5 
\  \   I 

OH  O 

3.  The  benzyl  ethers  of  the  two  benzaldoximes  showed  a 
characteristically   different  behavior  under  action  of  concen- 

i  Ber.  22,  613. 


126        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

trated  hydriodic  acid.  The  ether  of  a  benzaldehyde  gives 
ammonia  as  the  only  basic  separation  product,  while  the  benzyl 
group  is  separated  in  the  form  of  benzyl  iodide ;  from  the 
ether  of  ft  benzaldehyde,  benzylamine  is  formed  in  quantita- 
tive yield.  These  reactions  would  indicate  the  following 
constitutions :  — 

a  =  C6H5  -  CH  =  NOH  +  HI  =  C7H7I  +  NH8 

ft  =  C6H5  -  CH  -  NH    +.  HI  =  C7HrNH2 
\  x 
0 

On  the  other  hand,  various  arguments  have  been  brought 
forward  in  favor  of  the  structural  identity  of  these  two  bodies. 
They  undoubtedly  both  yield  the  same  oxidation  product,  and 
a  similar  argument  has  often  been  used  to  prove  the  structural 
identity  of  the  dioximes.  Goldschmidt1  has  shown  that 
when  treated  with  phenyl-iso-cyanide,  they  give  products 
which  must  be  represented  by  the  same  structural  formula. 
These  products  differ  only  slightly  in  their  properties,  and 
that  derived  from  the  iso-aldoxime  goes  over  with  the  greatest 
ease  into  the  form  derived  from  the  normal  aldoxime,  an  ease 
which  seems  incompatible  with  the  change  represented  in 
passing  from 

N-CO.NH  .C6H5 
C6H5-CH(  I 
N0 
to 

C6H6 .  CH  =  NO  -  CO  .  NH .  C6H5 

which  would  be  the  change  required  if  Beckmann's  formula 
for  the  iso-aldoxirne  be  accepted  as  correct. 

Moreover,  G-oldschmidt 2  found  that  by  some  modifications 
of  Beckmann's  methods,  he  could  in  several  cases  produce  the 

i  Ber.  22,  3109.  ;      2  Ber.  23,  2166. 


THE   STEREO-CHEMISTKY  OF  NITROGEN.  127 

same  ether  from  the  normal  and  iso-oximes,  and  he  concludes 
that  in  Beckmann's  experiments,  the  presence  of  water  may 
have  had  a  disturbing  effect  upon  the  smooth,  plain  course  of 
the  reaction. 

Hantzsch  and  Werner l  argue  that  the  formula 

C6H5-CH-NH 
\  / 
0 

is  untenable  for  iso-benzaldoxime,  because  that  would  seern  to 

be  an  intermediate  form  between 

O 

// 
C6HS .  CH  =  NOH        and        C6H5  -  C  -  NH2 

and  according  to  this,  the  iso-oxime  should  be  easily  changed 
into  benzamide,  while,  on  the  contrary,  it  may  be  directly 
decomposed  into  water  and  benzo-nitrile,  which  is  in  direct 
accordance  with  their  space  formula :  — 

C6H5-C-H 

1ST-OH 

They  also  urge  that  the  finding  of  an  ether  of  formula 

C6H6-CH.N.C,HT 

\  / 

O 

does  not  necessarily  prove  the  pre-formed  existence  of  the 
imido  group  in  iso-benzaldoxime.  They  think  that  the  oximes 
in  general  may  be  tautomeric  bodies,  reacting  in  the  sense  of 
two  formulas  with  groups 

C  =  N  -  OH 


OF  THE 

UNIVERSITY; 
/•%.,  o'L.k.ii., 


128        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

so  that  from  each  of  the  two  benzaldoximes,  an  ether  of  either 
formula  may  be  prepared,  but  that  the  normal  and  iso-oximes 
in  general  both  possess  the  structural  formula, 


V 

is  proved  by  the  fact  that  all  plain  reactions  carried  on  under 
absolute  exclusion  of  water  give  without  exception  derivatives 
of  the  structural  formula 


Meanwhile,  the  strife  in  regard  to  the  oximes  is  still  raging. 
Glaus1  and  Nef  2  are  strenuously  opposed  to  the  consideration 
that  the  oximes  in  general  have  been  proved  to  be  structurally 
identical,  although  so  many  facts  seem  to  point  in  that  direc- 
tion. Auwers,  Meyer,  and  Beckmann  admit  the  structural 
identity  of  most  of  the  oximes,  but  make  an  exception  in  the 
case  of  benzaldoxime,  while  Hantzsch,  Werner,  Goldschmidt, 
and  others  consider  it  proved  that  these  bodies  are  also  struc- 
turally identical,  although  they  may  react  as  tautomeric 
bodies. 

Granting  the  structural  identity  of  the  benzaldoximes,  the 
explanation  of  isomeric  oximes  on  the  theory  of  limited  rota- 
tion falls  to  the  ground.  On  the  assumption  that  they  are 
structurally  different,  the  Auwers  and  Meyer  theory  might 
stand  for  the  other  oximes,  but  even  then,  Hantzsch's  expla- 
nation would  seem  in  better  accordance  with  the  number  and 
properties  of  the  isomeric  forms  observed,  and  to  explain  on 

i  J.  fur  prak.  Chemie  [2],  44,  315  ;  45,  1,  377  ;  46,  34,  546,  556  ;  47,  267  ;  48, 
80. 

*  Liebig's  Annalen  der  Chemie,  270,  325. 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  129 

one  principle  the  isomerism  of  a  larger  number  of  compounds, 
without  necessitating  any  exception  to  the  van't  Hoff  hypothe- 
sis, which  is  found  to  be  true  in  such  a  vast  majority  of  cases  ; 
and  therefore  the  Hantzsch  and  Werner  theory  seems  most 
worthy  of  acceptance. 

Auwers  and  Meyer  themselves,  in  1890,1  accepted  as  valid 
the  criticisms  on  their  former  theory,  and  declared  their  sup- 
position  of  limited  rotation  untenable  a,s  an  explanation  of 
the  isomeric  nitrogen  compounds.  This  decision  was  has- 
tened by  the  discovery  of  certain  structurally  identical  ketox- 
imes  which  contained  no  singly  linked  carbon  atoms,  and 
which  therefore  could  not  be  explained  on  their  old  theory. 
Among  these  were  the  oximes  of  parachlor-benzo-phe- 
none,  C6H5  — CO  —  C6H4C1,  discovered  by  Auwers  and  Meyers 
themselves.2 

They  were,  however,  still  unwilling  to  accept  the  theory  of 
Hantzsch  and  Werner,  and  referred  the  phenomenon  instead 
to  a  stereo-isomerism  of  the  oxygen  atom.  Their  argument 
was  that  their  opponents  had  endeavored  to  explain  a  phe- 
nomenon found  only  in  hydroxylamine  by  reference  to  a 
property  of  the  nitrogen  atom,  and  they  could  not  therefore 
understand  why  a  similar  isomerism  had  not  been  found  in 
derivatives  of  ammonia  and  in  azo  and  azoxy  bodies.  So  long 
as  such  were  wanting,  they  concluded  that  the  cause  of  the 
phenomenon  should  be  found  not  in  the  nitrogen  atom,  but  in 
the  NH2OH  group.  The  formula  for  this  is  generally  written 
as  follows  :  —  *~^ 


Ber.  23, 2403.  2  Ber.  23, 2063. 


130        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

They  assumed,  however,  that  the  valencies  of  the  oxygen, 
like  those  of  all  other  atoms,  could  be  bent  from  their 
normal  direction,  and  that  the  hydrogen  of  the  hydroxyl 
would  take  a  position  compelled  for  it  by  the  attraction  both 
of  the  oxygen  and  nitrogen ;  that  is,  a  position  between  these 
two  atoms.  It  is  kept  from  drawing  nearer  to  the  nitrogen 
atom  in  a  straight  line  by  the  oxygen  which  lies  between,  and 
therefore  will  assume  the  position  represented  in  the  follow- 
ing formula,  in  which  it  is  assumed  that  the  hydrogen  of  the 
hydroxyl  is  not  in  the  plane  of  the  paper :  — 


The  formulas  for  the  two  oxiines  would  then  become 

a        H  a 

I  \  I 

Cr=N-O  and        C  =  N  -  0 
1  I  / 

b  b      H 

In  order  that  these  formulas  should  represent  two  different 
isomeric  forms,  it  is  evident  that  the  hydrogen  atom  must 
be  assumed  to  be  incapable  of  free  rotation  around  the  oxy- 
gen atom,  and  one  naturally  asks  the  reason  for  such  an 
assumption.  f 

New  discoveries  are  meanwhile  constantly  increasing  the 
number  of  isomeric  forms  which  can  be  explained  by  Hantzsch's 
hypothesis,  although  it  is  true  that  most  of  these  bodies  are 
such  as  may  be  considered  as  derivatives  of  hydroxylamine, 
including  the  aldoxiines  and  aldoxime  carbonic  acids,  ket- 
oximes,  dioximes,  and  the  derivatives  of  hydroxamic  acid. 


THE   STEREO-CHEMISTRY  OF  NITROGEN.  131 

In  these  compounds,  the  change  of  one  isomer  to  the  more 
stable  form  takes  place  usually  under  the  influence  of  heat, 
but  sometimes  gradually  of  itself  at  ordinary  temperatures, 
and  quite  commonly  under  chemical  influences.  If  the  rise 
of  temperature  goes  on  to  the  decomposition  of  the  molecule, 
the  same  decomposition  products  are  formed  from  each  isomer. 
In  these  respects  the  oximes  are  closely  analogous  to  the 
ethylenic  bodies,  and  this  analogy  is  shown  by  Hantzsch's 

The  aldoximes  are  of  two  kinds,  —  those  which  decompose 
easily  into  water  and  a  nitrile,  and  those  which  do  not  undergo 
this  decomposition.  These  forms  are  explained  by  Hantzsch's 
formulas  as  follows  :  — 

R_C-H  R-C-H 

II  and  II 

N-OH  HO-N 

(1)  —  syn  aldoxime.  anti  aldoxime. 

The  stability  is  much  influenced  by  R.  In  the  fatty  king- 
dom, the  syn  aldoximes  are  almost  the  only  ones  capable  of 
existence,  while  in  the  benzene  series  the  anti  prevail.  In 
cases  where  only  one  isomeric  form  is  known,  this  one  corre- 
sponds to  one  or  the  other  of  the  classes  named  above,  and  it 
is  supposed  that  its  stereo-isomeric  modification  is  too  unstable 
to  have  a  lasting  existence.  This  stereo-isomerism  is  very 
common  in  the  aromatic  series,  but  has  been  observed  in 
only  a  few  of  the  fatty  derivatives  ;  e.  g.,  in  the  aldoxime, 


CH3-CH  = 

in  aldoximacetic  acid, 

H 
/ 
COOH  -  CH2  -  C  =  NOH, 


132         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

and  in  a  few  other  compounds  recently  described  by  Dunstan 
and  Dymond.1 

The  ketoximes  are  very  similar  to  the  aldoximes;  but  in 
these  bodies,  the  isomeric  forms  resemble  each  other  more 
closely  than  is  the  case  with -the  aldoximes,  on  account  of  the 
greater  similarity  of  the  groups  connected  with  the  carbon. 
These  isomeric  forms  are  also  distinguished  as  syn  and  anti} 
according  to  the  following  formulas  :  — 

C6H5  -  C  -  C6H4CH3  C6H6  -  C  -  C6H4'.  CH3 

II  and  II 

HO-N  N-OH 

Syn  phenyl  tolyl  ketoxime.  Anti  phenyl  tolyl  ketoxime. 

For  distinguishing  between  these,  the  Beckmann  reaction  is 
made  use  of ;  viz.,  the  transformation  of  the  oxime  into  the 
structurally  isomeric  acidamide.  These  reactions  are  indi- 
cated in  the  following  formulas  :  — 

Kj-C-R,  H-C-R2  O=C-Rs 

II  ->  II  ->  I 

HO-N  RiO-N  RiH  =  N 

Ri-C-R,,       ->        Ri-C-H       -*       ^-C  =  0 
II  II  I 

N-OH  N-OR2  N  =  R2H 

Illustrations  of  syn  and  anti  derivatives  of  hydroxamic 
acids  may  be  found  in  the  ethyl  benzhydroxamic  acids, 

C6H,  -  C  -  OC2H5  C6H5  -  C  -  OC2H6 

II  and  II 

HO-lsT  N-OH 

syn  anti 

i  J.  Ch.  S.  65,  206.    : 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  133 

In  the  case  of  the  dioximes,  three  isomeric  forms  are  pos- 
sible according  to  Hantzsch's  formulas,  and  this  number 
agrees  exactly  with  the  number  of  benzil  dioximes  which  have 
been  discovered.  The  formulas  for  these  three  dioximes  have 
already  been  given,  (p.  123),  and,  adopting  the  nomenclature  of 
the  other  oximes,  the  /?  form  is  the  syn  dioxime,  y  would  be 
designated  as  anti,  and  a  as  amphi. 

An  isomerism  in  the  hydrazones,  apparently  not  due  to 
structural  differences,  was  first  observed  by  Fehrlin1  and 
Krause 2  in  phenyl-hydrazone  of  o  nitrophenyl  glyoxalic  acid. 
Since  then,  several  other  similar  cases  have  been  described. 
These  are  explained  by  Hantzsch 8  on  exactly  the  same  prin- 
ciple as  the  oximes,  the  formulas  for  the  two  phenyl-hydra- 
zones  of  anisyl-phenyl-ketone  being  written 

C6H5  -  C  -  C6H4OCH3  C6H5  -  C  -  C6H4OCH3 

II  and  II 

C6H5  .  HN-N  N-NH.  C6H5 

These  are  all  cases  of  compounds  in  which  nitrogen  with  a 
valence  of  III  is  joined  by  double  bonds  to  carbon.  That  the 
same  kind  of  isomerism  may  be  observed  when  pentad  nitro- 
gen is  doubly  linked  to  carbon  may  be  inferred  from  the 
apparent  retention  of  this  isomerism  in  some  salts  of  the 
oximes, 

a.  /H 

;C  =  N-OH 

b'  \C1 

but  these  bodies  have  not  as  yet  received  a  very  thorough 
study. 

There  remains  to  be  considered  only  the  compounds  em- 
braced under  II.  b,  those  in  which  two  atoms  of  triad  nitrogen 

i  Ber.  23,  1574.  2  Ber.  23,  3617.  «  Ber.  26,  9. 


134        THE   DEVELOPMENT  OF   STEREO-CHEMISTRY. 

are  doubly  linked  to  one  another.  It  is  quite  conceivable  that 
there  may  be  a  stereo-isomerism  in  these  bodies,  analogous  to 
that  observed  in  bodies  of  the  type 

\  / 

C 
II 

N 
\ 

and  represented  by  the  formulas 

N  — a  N  —  a 

II         and          II 

a— N  N  — j 

Well-authenticated  illustrations  of  such  an  isomerism,  how. 
ever,  have  been  lacking  until  quite  recently,  and  it  has  been 
considered  doubtful  whether  such  an  isomerism  did  not  in 
reality  exist,  or  whether  the  lack  of  the  observed  phenomenon 
was  to  be  attributed  simply  to  the  fact  that  bodies  of  this 
type  have  in  general  a  rather  complicated  structure,  and  have 
not  been  very  thoroughly  investigated.  It  has  been  sug- 
gested, however,  that  this  theory  may  serve  to  explain  the 
two  tri-nitro-azo-toluenes  which,  according  to  Janovski,  have 
the  same  structure  :  — 

N-C6H3-N02.CH3 

N-C6H2(N02)2.CH3 

and 

CH3.N02.C6HS-N 

N-C6H2(N02)2CH3 

Within  the  present  year  (1894),  Hantzsch  l  has  described 
some  stereo-isomeric  diazo  and  diazo  amido  bodies  which  seem 
i  Ber.  27,  1702,  1726-1857. 


THE  STEEEO -CHEMISTRY  OF  NITROGEN.  135 

to  belong  in  the  class  under  consideration.  As  one  illustra- 
tion, may  be  mentioned  the  diazo  sulphonates  which  he  repre- 
sents by  the  following  formulas  :  — 

Ph-N  Ph-N 

II  II 

SCXK-N  N-SOQK 


Potassium  benzene  syndiazo  Potassium  benzene  anft'diazo 

sulphonate.  sulphonate. 

Several  other  bodies  of  this  type  have  been  shown  by  him 
to  exhibit  the  same  kind  of  isomerism,  so  that  it  seems  prob- 
able that  two  doubly  linked  nitrogen  atoms  may  assume  con- 
figurations in  space  closely  analogous  to  those  exhibited  by 
two  doubly  linked  carbon  atoms. 

Independently  of  any  conception  as  to  the  geometrical  form 
of  the  nitrogen  atom  itself,  the  simplest  and  most  natural 
assumption  as  to  the  arrangement  of  a  molecule  containing 
triad  nitrogen  is  that  the  nitrogen  atom  is  situated  in  the 
centre,  with  the  three  combining  atoms  or  radicals  arranged 
around  it  symmetrically  in  a  plane,  and  this  assumption  is  in 
accordance  with  the  facts  that  triad  nitrogen  compounds  are 
characterized  by  stability  and  an  absence  of  isornerism. 


If,  however,  the  nitrogen  atom  be  joined  to  the  carbon 
tetrahedron  by  1  and  2,  it  is  quite  conceivable  that  3  should 
be  drawn  from  the  plane  of  the  paper  by  its  attraction  for  one 


136         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

or  the  other  of  the  two  radicals  attached  to  the  carbon,  accord- 
ing to  Hantzsch  and  Werner's  theory,  giving  a  result  similar 
to  what  would  have  been  the  case  if  the  N  occupied  one  angle 
of  a  tetrahedron,  and  1,  2,  and  3  the  other  three  angles. 

When  the  two  dormant  valencies  of  nitrogen  are  called  into 
play,  the  problem  becomes  more  complicated,  but  the  simplest 
assumption  is  that  these  two  are  symmetrically  arranged  with 
reference  to  the  other  three.1 


This  would  make  4  and  5,  one  below  and  the  other  above  the 
plane  of  the  paper.  Professor  Pickering  calls  attention  to  the 
fact  that  the  law  of  symmetry  would  be  satisfied  if  4  and  5 
were  the  only  valence-places  saturated.  With  monad  ele- 
ments, this  would  probably  never  occur,  since  they  would 
naturally  take  places  1,  2,  and  3;  but  with  a  bivalent  element, 
it  would  be  quite  conceivable  that  4  and  5  should  alone  be 
saturated,  and  thus  we  get  a  plausible  explanation  of  such 
bodies  as  NO. 

The  above  formula  is  plainly  not  perfectly  symmetrical, 
which  agrees  with  the  fact  that  there  is  a  tendency  for  two 
atoms  or  radicals  to  split  off  and  leave  the  more  symmetrical 
triad  nitrogen. 

Objections  have  been  urged  against  the  explanation  of  opti- 
cal activity,  in  the  completely  asymmetric  nitrogen  molecule, 
by  the  assumption  that  (N~H)IV  may  take  the  place  of  CIV  in  a 

1  See  Professor  Pickering.    J.  Chera.  Soc.,  Sept.,  1893. 


THE  STEREO-CHEMISTRY  OF  NITROGEN.  137 

tetrahedron,  but,  adopting  the  above  formula,  it  will  be  seen 
that  something  entirely  analogous  to  this  is  possible.  If  the 
nitrogen  is  supposed  to  occupy  the  centre  of  the  molecule,  and 
3  be  a  hydrogen  atom,  and  1,  2,  4,  and  5  be  replaced  by  four 
different  radicals,  then  lines  joining  the  last  four  points  would 
give  the  figure  of  an  irregular  tetrahedron  entirely  analogous 
to  the  asymmetric  carbon  tetrahedron. 

It  will  be  observed  that  the  figure  just  discussed  is  practi- 
cally the  same  as  that  of  Burch  and  Marsh. 


V. 


VARIATIONS  IN  OPTICAL    ACTIVITY    AND    RELA- 
TIONS  OF  STEREO-CHEMISTRY   TO  CRYS- 
TALLOGRAPHY. 

THOUGH  there  is  much  in  connection  with  the  subject  of 
stereo-chemistry  which  is  still  in  a  vague  and  unsettled  state, 
though  there  are  many  unanswered  questions,  and  so  much 
new  matter  constantly  being  brought  forward  to  be  sifted  and 
explained  that  we  must  feel  that  our  knowledge  of  this 
branch  is  still  quite  in  its  infancy,  yet  it  has  already  done 
much  for  the  parent  science  of  Chemistry.  If  in  no  other 
way,  it  has  done  great  service  in  introducing  symbols  and 
formulas  which  explain  better  the  observed  properties  of 
organic  bodies  than  any  which  have  previously  been  used. 
It  has  also  stimulated  research,  and  carefulness  and  delicacy 
of  work  in  the  preparation  of  many  new  organic  compounds, 
in  improved  methods  of  preparing  and  purifying  bodies  long 
known,  and  in  a  more  careful  study  of  certain  of  their  proper- 
ties. This  is  especially  noticeable  in  connection  with  the  sub- 
ject of  optical  activity.  It  has  already  been  mentioned  that 
when  van't  HofFs  theory  was  first  announced,  various  bodies 
were  cited  as  showing  optical  activity  without  asymmetric 
carbon.  To  settle  this  point  it  was  of  course  essential  that 
the  substances  should  be  in  a  state  of  absolute  purity,  and 
their  optical  activity  determined  with  the  greatest  care,  thus 


VARIATIONS  IN  OPTICAL  ACTIVITY.  139 

leading  to  much  careful  experimentation  before  van't  HofPs 
hypothesis  could  be  verified. 

By  the  study  of  stereo-chemistry,  the  subject  of  optical 
activity  has  gained  a  new  importance  and  interest  on  account 
of  the  analogy  in  structure  thus  suggested  between  certain 
crystalline  forms  and  some  organic  molecules.  Since  it  is 
found  that  the  rotatory  polarization  exhibited  by  quartz  and 
other  crystalline  bodies  is  also  shown  in  the  solutions  of  cer- 
tain organic  compounds,  we  have  an  indication  that  these 
organic  molecules  are  structures  whose  atoms  have  a  definite 
and  fixed  arrangement  analogous  to  the  arrangement  of  the 
molecules  in  the  crystals.  Any  new  facts,  therefore,  in  regard 
to  the  optical  activity  of  organic  liquids  are  to  be  welcomed 
in  the  hope  that  they  may  ultimately  throw  more  light  upon 
molecular  structure.  Communications  on  this  subject  are  con- 
stantly being  published  in  the  various  scientific  journals,  some 
of  them  relating  to  the  optical  activity  of  particular  bodies, 
others  of  a  more  general  nature,  and  the  subject  is  of  suffi- 
cient importance  to  warrant  a  brief  allusion  here  to  some  of 
the  most  recent  investigations  in  this  field. 

Some  of  the  most  important  of  these  investigations  have 
been  in  regard  to  the  variations  which  optical  activity  under- 
goes under  different  conditions  as  to  solvent,  temperature,  etc. 
That  each  substance  has  its  own  characteristic  amount  of 
rotatory  power  is  true  if  the  conditions  remain  fixed,  but  the 
amount  varies  very  much  under  varying  circumstances.  It  is 
well  known,  for  example,  that  the  amount  of  deviation  in- 
creases proportionally  with  the  length  of  the  column  of  liquid 
examined. 

As  early  as  1838,  Biot  had  observed  in  a  few  cases  that, 
with  solutions  of  the  same  concentration  of  a  particular  sub- 
stance, the  amount  of  rotatory  power  varied  with  the  solvent 


140        THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

employed.  These  observations  were  for  some  time  overlooked, 
but  in  1873,  A.  C.  Oudemans  undertook  some  experiments  to 
determine  the  effect  of  different  solvents  on  the  rotatory 
power  of  organic  bodies,  and  found  that  although  some  bodies, 
as  sugar,  for  example,  did  not  alter  essentially  with  the  various 
solvents,  others,  especially  the  alkaloids  and  their  salts,  suffer 
considerable  deviations. 

In  1893  Freundler l  undertook  some  experiments  to  deter- 
mine the  influence  of  various  organic  solvents  on  the  rotatory 
power  of  certain  ethers  of  tartaric  acid.  Referring  here  only 
to  the  two  solvents  tried  which  gave  the  greatest  deviation 
from  each  other,  his  results  are  as  follows  :  — 


Solvent. 

.Solvent. 

With  diacetyl  tartrate 

Carbon  disulphide 

+36.7° 

Bromoform 

-2.6° 

"      dipropionyl  tartrate 

«i                             4t 

+35.5° 

« 

-2.0° 

"      dibutyryl  tartrate 

«                            <t 

+28.8° 

" 

-3.8° 

"      di  n  valeryl  tartrate 

Acetone    .    .    . 

+  8.2° 

M 

-4.7° 

"     di  n  caproyl  tartrate 

Methyl  alcohol  . 

+  5.4° 

Benzene  . 

-2.5° 

Preundler  found,  in  general,  that  oxygenated  solvents  made 
little  or  no  change  in  the  amount  of  rotatory  power,  while  the 
halogen  compounds  lower  it  much,  even  to  changing  of  sign, 
as  shown  in  the  above  results.  If  the  optical  activity  is  to  be 
ascribed,  as  has  been  supposed,  to  molecular  structure,  it  is 
evident  that  different  solvents,  none  of  which  exert  any  action 
on  the  molecule,  should  not  alter  the  activity  of  the  substance. 
It  has  been  suggested,  however,  that  the  variations  in  these 
cases  may  be  due  either  to  polymerization  or  to  combination 
of  the  solvent  with  the  active  body.  Freundler  concludes,  as 


Compt.  rend.  117,  556. 


VARIATIONS  IN  OPTICAL  ACTIVITY.  141 

the  result  of  his  researches,  that  normal  values  of  deviation 
are  obtained  if  the  molecular  weight  remains  unchanged,  and 
that  abnormal  values  are  obtained  only  with  solvents  which 
act  upon  the  dissolved  ethers  j  therefore,  his  investigations 
really  cast  no  doubt  upon  the  constancy  of  the  property  in  an 
unaltered  active  compound. 

That  the  amount  of  rotatory  power  of  an  active  substance 
is  dependent  on  the  temperature  is  a  fact  which  has  long  been 
known,  but  it  is  only  recently  that  this  variation  has  been 
systematically  studied.  In  Landolt's  monograph,  "  Das  op- 
tische  Drehungsverrnogen  organischer  Substanzen,"  published 
in  1879,  it  is  stated  that,  so  far  as  then  known,  increase  in 
temperature  is  generally  associated  with  diminution  of  rota- 
tory power.  Landolt  himself  found,  however,  that  the  rota- 
tory power  of  nicotine  increases  slightly  with  the  temperature  ; 
the  same  has  been  found  true  in  regard  to  certain  ethereal 
salts  of  tartaric  acids ;  and  Frankland  and  MacGregor  1  have 
observed  the  same  with  some  glycerates  and  diacetyl  glycer- 
ates,  so  that  there  seeing  to  be  at  present  no  general  rule  in 
regard  to  this  variation.  The  paper  of  Frankland  and  Mac- 
Gregor, just  cited,  closes  as  follows :  "  The  results  of  our 
experiments  as  well  as  those  of  others  clearly  show  that  the 
effect  of  temperature  on  optical  activity  will  have  to  be  more 
taken  into  consideration  in  the  future  than  it  has  been  in  the 
past,  and  when  systematically  studied  for  a  large  number  of 
active  substances,  it  may  assist  in  throwing  light  on  the  inter- 
nal arrangement  of  the  molecules  of  active  compounds,  for  it 
has  already  been  ascertained  in  several  cases  that  the  influ- 
ence of  temperature  on  rotation  is  independent  of  any  mole- 
cular polymerization." 

The  possibility  of  change  of  temperature  producing  poly- 
*  J.  Ch.  Soc.  66,  760. 


142         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

meric  changes,  and  thus  causing  optical  variations,  has  been 
occasionally  brought  forward  as  an  answer  to  those  who  urged 
that  these  variations  in  rotatory  power  with  the  temperature 
were  an  argument  against  this  power  depending  in  any  way 
upon  space  relations.  To  give  a  concrete  illustration,  Colson,1 
in  1893,  undertook  some  experiments  with  active  bodies  under 
varying  conditions  of  temperature.  The  following  are  the 
results  obtained  with  the  oxide  of  iso-butyl-iso-amyl,  in  a  tube 
0.20  m.  long,  and  the  deviations  observed  with  reference  to 
ray  D :  — 

Temperature.  Angle  of  deviation. 

-40°  -0°     & 

—21°  —0°     4' 

-  4°  +0°  11' 

+15°  +0°  13' 

+40°  +0°  15' 

It  will  be  observed  that  in  passing  from  —  40°  to  +  40°,  the 
angle  of  deviation  has  varied  by  21',  changing  sign  between 
—  21°  and  —  40°.  Colson  uses  these  observations  as  an  argu- 
ment against  the  theory  that  the  rotatory  power  depends  in 
any  way  upon  space  relations,  arguing  that  if  the  deviation 
were  caused  by  the  position  in  space,  the  rotatory  power 
should  not  be  influenced  by  physical  causes  incapable  of 
changing  the  nature  and  state  of  the  body  ;  by  cooling  a  body 
without  freezing  it,  for  example,  this  should  remain  constant. 
He  therefore  concludes  that  the  chemical  constitution  cannot 
be  the  factor  preponderating  in  the  value  or  in  the  sign  of  the 
rotatory  power. 

As  an  answer  to  this,  it  was  urged  that  changes  of  tempera- 
ture may  make  changes  of  the  nature  of  polymerization  in  the 
aggregate  which  constitutes  the  optical  molecule,  and  thus 
1  Compt.  rend.  116,  319. 


VARIATIONS  IN  OPTICAL  ACTIVITY.  143 

cause  these  variations.  Bamsay  has,  however,  found  that 
some  substances  whose  rotatory  power  does  not  vary  with  the 
temperature  do  polymerize,  and  that  others,  whose  rotatory 
power  is  variable,  do  not  polymerize.  It  would  seem,  then, 
that  the  variations  must  be  due  to  internal  changes  in  the 
molecule.  Le  Bel1  states  that  all  the  compounds  actually 
known  to  have  variable  rotatory  power  are  simple  ethers  ;  i.e., 
compounds  in  which  the  asymmetric  carbon  is  united  only 
with  a  single  radical  containing  an  atom  of  oxygen  attached 
to  another  radical.  When  the  asymmetric  carbon  is  united 
with  two  radicals  of  similar  constitution,  the  variations  of 
rotatory  power  become  almost  nil,  indicating  that  two  radicals 
of  the  same  nature  compensate  for  each  other.  Le  Bel  thinks 
these  facts  can  be  explained  only  on  the  assumption  that  the 
univalent  unions  become  immobile  at  low  temperatures,  the 
molecule  undergoing  a  kind  of  internal  congelation,  but  they 
become  mobile  again  at  higher  temperatures,  where,  indeed,  it 
is  observed  that  the  rotatory  power  tends  to  become  constant, 
which  seems  to  indicate  that  perfect  mobility  might  then  be 
attained. 

All  this  study  of  optical  activity  shows  that  it  is  a  property 
which  is  very  easily  influenced  by  external  conditions,  but 
exactly  how  or  why  these  conditions  affect  it  are  points  which 
are  not  yet  very  satisfactorily  explained.  The  most  that  can  be 
said  at  present  is  that  there  is  nothing  in  these  variations  to 
cast  any  serious  doubts  upon  the  fundamental  principles  of 
stereo-chemistry. 

Fock 2  has  published  some  interesting  speculations  in  regard 
to  the  cause  of  optical  activity,  accepting  van't  Hoff's  tetra- 
hedral  theory  as  his  starting-point.  He  recalls  the  fact  that 
the  rotation  of  the  plane  of  polarization  consists  in  the  divi- 

i  Compt.  rend.  118,  916.  2  Ber.  24,  101. 


144        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

sion  of  the  entering  ray  of  plane  polarized  light,  in  the  active 
medium,  into  two  circular  rays,  vibrating  in  opposite  direc- 
tions and  with  different  velocities,  which,  on  leaving  the  sub- 
stance, are  again  compounded  into  one  linearly  polarized  ray. 
Fleischl  has  discovered  for  liquids  that  in  the  active  sub- 
stances there  is  also  a  true  double  refraction.  Another  char- 
acteristic of  these  optically  active  substances  is  that  when 
they  crystallize,  they  always  crystallize  in  hemihedral  forms, 
and  this,  in  the  generally  accepted  theories  of  crystallogra- 
phers,  means  that  a  polarity  must  be  ascribed  to  the  molecules 
of  the  hemihedrally  crystallized  substance.  These  facts,  in 
the  opinion  of  Fock,  are  held  together  and  explained  by  tak- 
ing as  axis  of  polarity  the  rotation  axis  of  the  molecule,  which 
is  determined  by  a  straight  line  passing  through  the  centre  of 
gravity  of  the  molecule,  and  that  tetrahedral  angle  to  which 
is  attached  the  radical  of  greatest  mass.  In  the  accompany- 
ing cut,  if  a  is  the  radical  of  greatest  mass,  and  if  b  >  c  >  d, 
then  this  molecule  will  find  less  resistance  of  the  ether  rotat- 
ing in  that  direction  than  in  the  reverse. 


This  would  seem  to  offer  a  possible  explanation  of  the  differ- 
ent velocities  of  the  two  circularly  polarized  rays  in  opposite 
directions.  He  concludes  that  a  rotatory  motion  of  the  mole- 
cule only  produces  a  rotation  of  the  plane  of  polarization 
under  the  following  conditions :  — 

1.  The  polarity  of  the  molecule  must  be  exhibited  in  such  a 
way  that  a  ray  of  light  will  find  a  greater  resistance  moving 
in  one  direction  around  the  rotatory  axis  than  in  the  other. 


VARIATIONS  IN  OPTICAL  ACTIVITY.  145 

2.  The  rotation  of  the  molecule  must  take  place  either  to 
the  right  or  left  around  this  axis. 

These  two  conditions  can  be  met  with  only  in  molecules 
built  up  around  asymmetric  carbon.1 

One  of  the  most  interesting  ways  in  which  stereo-chemistry 
is  suggestive,  without  offering  any  definite  enlightenment,  is 
in  indicating  relations  between  molecular  and  crystalline 
structure.  It  has  already  been  pointed  out  that  the  idea  of 
enantiomorphous  forms  in  the  invisible  molecules  is  derived 
from  the  hemihedral,  enantiomorphous  forms  seen  in  crystals, 
and  this  because  the  crystals  with  spiral  arrangement  of  hemi- 
hedral  faces  share  the  property  of  rotatory  polarization  with 
molecules  possessing  no  plane  of  symmetry,  and  which  may 
be  considered  as  built  up  around  asymmetric  carbon  with  a 
spiral,  or  circular,  arrangement  of  the  atoms  either  to  the  left 
or  right.  The  representation  of  molecules  by  regular  geo- 
metrical forms  in  three-dimension  space,  which  stereo-chem- 
istry has  introduced,  cannot  fail  to  suggest  at  once  the  thought 
of  a  crystal,  and  to  lead  to  the  hope  that  by  a  more  thorough 
and  systematic  investigation  of  the  forms  of  molecules,  it  may 
be  possible  to  trace  some  connection  between  these  forms  and 
those  of  the  corresponding  crystallized  substances. 

The  laws  which  determine  the  crystalline  form  which  each 
particular  substance  shall  assume  are  as  yet  beyond  our  grasp ; 
^but  there  are  many  facts  to  show  that  this  is  influenced  by  the 
complexity  of  the  molecule,  and  also  that  each  individual 
atom  plays  its  part  in  some  way  in  determining  the  shape  of 
the  crystal.  Thus  it  has  been  shown  that  a  large  proportion 
of  the  elements  and  simple  inorganic  compounds  crystallize 
in  the  regular  and  hexagonal  systems,  while  complex  inor- 
ganic and  organic  compounds  crystallize  more  frequently  in 

1  For  Ostwald's  criticism  of  this  work,  see  Zeit.  f.  pr.  Chem.  7,  429. 

10 


146         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

rhombic  and  nionoclinic  systems.  This  indicates  a  connec- 
tion between  simplicity  of  molecular  form  and  the  largest 
possible  amount  of  regularity  or  symmetry  in  crystalline 
structure. 

The  fact  that  each  constituent  atom  of  the  molecule  may 
also  affect  the  crystalline  symmetry  of  the  body  has  been 
shown  by  Groth  and  others,  who  have  studied  the  effect  of 
the  substitution  of  different  elements  in  the  molecule  of  ben- 
zene derivatives,  and  have  shown  that  such  substitution  is 
always  accompanied  by  a  definite  change  in  the  crystalline 
form.  Thus,  the  substitution  of  hydrogen  by  OH  or  N02 
leads  to  changes  in  the  relations  of  the  axes  to  one  another, 
but  not  to  a  change  of  crystallographic  system ;  but  the  sub- 
stitution of  hydrogen  by  chlorine  or  bromine  changes  the 
system  to  one  less  symmetrical.  This  last  observation  is 
hardly  what  might  have  been  expected,  since  the  substitu- 
tion of  one  element  by  another  would  not  increase  the  com- 
plexity of  the  molecule,  unless 'indeed  this  may  be  taken  as 
another  indication  of  the  compound  nature  of  the  halogen 
elements. 

It  would  seem  as  if  the  facts  of  isomorphism  might  be 
expected  to  throw  more  light  on  the  connection  between  mo- 
lecular and  crystalline  structure  than  is  found  to  be  the  case. 
One  atom  may  be  replaced  by  another  atom  or  group  of  atoms, 
having  the  same  valence  as  itself,  without  changing  the  crys- 
talline form  of  the  compound,  but,  on  the  other  hand,  another 
atom  of  the  same  valence  may,  on  its  substitution,  completely 
change  the  form  of  the  crystal.  There  are  two  ways  in  which 
this  latter  observation  may  be  explained.  Either  the  new 
atom  gives  a  new  shape  to  the  molecule,  or  it  influences  the 
number  of  simple  molecules  which  go  to  make  up  the  crystal- 
line unit ;  and  since  the  valence  remains  the  same,  the  former 


VARIATIONS  IN  OPTICAL  ACTIVITY.  147 

explanation  seems  at  first  sight  a  more  natural  one  than  the 
latter.  This  would  lead  to  the  idea  of  the  crystalline  form  as 
depending  very  closely  on  the  shape  of  the  simple  molecules 
of  which  it  is  composed,  and  it  was  on  an  assumption  of  this 
kind  that  Le  Bel  based  his  objection  to  the  tetrahedral  symbol 
of  carbon. 

The  difference  between  the  point  of  view  of  Le  Bel  and  that 
of  van't  Hoff  has  already  been  briefly  alluded  to.  In  1890, 
Le  Bel 1  again  called  attention  to  these  differences,  and  stated 
his  objections  to  the  tetrahedral  idea.  He  assumes  that  if 
CR4  has  necessarily  the  geometrical  form  of  a  regular  tetra- 
hedron, then  it  follows  that  all  bodies  of  that  general  formula 
should  crystallize  in  the  cubic  system,  and  that  C(B,)3R,  should 
be  rhoinbohedral.  These  conclusions  he  does  not  find  verified 
by  facts.  CBr4,  for  example,  he  finds  does  not  crystallize  in 
the  regular  system,  and  there  are  other  similar  discrepancies, 
from  which  he  concludes  that  CR>4  need  not  always  have  the 
form  of  a  regular  tetrahedron,  but  that  two  forms  are  pos- 
sible for  a  molecule  of  this  type.  These  two  forms  he  derives 
by  reference  to  repulsive  spheres  around  the  atoms,  the  ar- 
rangement of  these  spheres  depending  on  the  chemical  nature 
of  the  radical  B-. 

The  study  of  vapors  has  shown  that  the  attraction  of  mole- 
cules ceases  when  they  approach  very  near  each  other,  and 
gives  place  to  a  kind  of  repulsion,  as  if  the  molecule  were  sur- 
rounded by  a  sphere,  within  which  active  repulsions  exist. 
Now  it  is  very  probable  that  inside  the  molecule  there  are 
analogous  repulsions  between  the  atoms,  and  that  such  repul- 
sive forces  play  a  certain  part  in  the  equilibrium  of  the  mole- 
cule. In  the  case  of  an  atom  confined  in  a  molecule,  the 
oscillations  probably  do  not  take  place  with  equal  ease  in  all 
i  Bull.  Soc.  Chim.  [3],  3,  788. 


148        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

directions,  and  therefore  the  repulsive  zone  will  not  be  a  per- 
fect sphere,  but  the  same  general  reasoning  can  be  applied  as 
if  it  had  a  spherical  form,  and  therefore  the  term  "  repulsive 
sphere  "  is  retained. 

Applying  these  ideas  to  the  formula  CE4,  and  considering 
first  only  the  relations  between  C  and  E,  each  of  the  radicals 
will  be  drawn  toward  the  carbon  till  its  repulsive  sphere 
touches  that  of  the  latter,  when  there  is  developed  enough 
repulsive  force  to  bring  them  to  equilibrium,  on  the  condition 
that  the  reciprocal  action  between  the  radicals  does  not  oppose 
this.  With  the  radicals  themselves,  on  the  same  principle, 
any  two  of  them  may  approach  each  other  until  their  repul- 
sive spheres  touch,  and  this  gives  the  regular  tetrahedron  as 
the  limit  of  this  system  of  equilibrium.  It  is,  however,  pos- 
sible that  the  action  of  the  carbon  may  predominate  over  the 
reciprocal  action  of  the  radicals,  causing  their  repulsive 
spheres  to  interpenetrate  to  a  certain  extent,  and  this  gives 
another  possibility  of  equilibrium  in  a  second  geometrical 
form. 

Le  Bel  has  thus  accounted  for  the  possible  existence  of  two 
different  geometrical  forms  for  the  molecule  CR4,  and  there- 
fore, on  the  assumption  that  the  crystalline  form  must  possess 
the  same  type  of  symmetry  as  the  simple  chemical  molecule, 
he  has  also  accounted  for  the  fact  that  bodies  of  the  type  CR4 
may  crystallize  in  stwo  different  systems ;  but  we  cannot  re- 
frain from  asking  if  such  an  explanation  was  absolutely  neces- 
sary. Assuming  that  the  carbon  atom  occupies  the  centre  of 
a  sphere,  and  that  its  four  attendant  radicals  oscillate  about 
mean  positions  which  bear  to  one  another  the  relations  of  the 
four  angles  of  a  tetrahedron  inscribed  in  this  sphere,  does  it 
necessarily  follow  that  the  accumulation  of  such  molecules 
under  the  influence  of  the  crystallizing  force  must  arrange 


VARIATIONS  IN  OPTICAL  ACTIVITY.  149 

themselves  in  some  form  of  the  regular  system  ?  Though 
this  seems  at  first  glance  a  natural  outcome  of  the  assumption, 
it  must  be  granted  that  the  particular  shape  of  the  crystalline 
molecule  must  depend  on  the  number,  no  less  than  the  form, 
of  the  simple  molecules  which  go  to  make  it  up,  and  can  be 
conceived  as  belonging  to  a  different  type  of  symmetry  from 
that  of  the  simple  molecule.  How  else  can  the  phenomenon 
of  dimorphism  be  explained?  To  explain  the  existence  of 
orthorhornbic  and  rhombohedral  calcium  carbonate,  one  must 
either  assume  that  the  simple  molecule  CaC08  may  have 
different  forms  under  different  circumstances,  or  that  the  num- 
ber of  these  molecules  in  the  crystalline  unit  may  vary ;  and 
the  latter  seems  the  more  plausible  explanation  of  the  two. 
If  this  can  explain  the  existence  of  CaC03  in  two  crystalline 
forms,  why  is  it  not  sufficient  to  explain  the  fact  that  CE4 
does  not  always  crystallize  in  the  same  system? 

Certain  points  in  regard  to  isomorphism  indicate  that  the 
crystalline  form  cannot  be  considered  as  a  direct  result  of 
some  inherent  property  of  the  constituent  atoms,  such  as  their 
form.  Manganese,  for  example,  may  be  isomorphous  with 
chlorine,  sulphur,  aluminum,  or  copper,  according  to  the  func- 
tion which  it  has  in  the  particular  molecule,  and  yet  the  form 
of  the  manganese  atom  is  generally  assumed  to  be  unalterable. 
When,  therefore,  two  elements  which  are  usually  isomorphous 
are  found  in  corresponding  compounds  which  do  not  exhibit 
isomorphism,  as,  for  example,  the  nitrates  of  sodium  and  potas- 
sium, it  is  most  probable  that  the  configurations  of  the  sim- 
ple molecules,  as  KN03  and  NaN03,  must  be  the  same,  and 
the  difference  in  crystalline  form  must  then  be  attributed  to 
some  outside  cause,  as,  for  example,  to  the  number  and  ar- 
rangement of  the  simple  molecules  constituting  the  crystalline 
unit. 


150        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

Recently  an  attempt  has  been  made  by  Fock *  to  trace  some 
connection  between  the  tetrahedral  form  of  the  carbon  atom, 
and  the  crystallographic  form  of  certain  compounds  contain- 
ing carbon.  He  considers  one  of  the  positive  results  of  stereo- 
chemical  investigation  to  be  that  the  affinities  of  the  atoms 
have  not  only  a  definite  value,  but  also  a  definite  direction,  and 
this  suggests  a  correlation  between  the  affinity  directions  of  the 
atoms  and  the  different  directions  in  the  crystal  in  which 
different  properties  are  observed.  The  most  simple  and  nat- 
ural correlation  here  conceivable  is  that  the  crystalline  mole- 
cule and  the  crystal  itself  should  possess  the  same  type  of 
symmetry,  but  it  is  generally  conceded  that  the  crystalline 
molecule  is  a  larger  unit  than  the  chemical  molecule,  and  its 
exact  size  is  unknown.  However,  Fock  proceeds  to  show  that 
the  explanation  of  the  crystalline  form  from  the  chemical 
composition  is  not  impossible,  though  until  further  investiga- 
tion gives  some  clew  to  the  actual  number  •  of  atoms  in  the 
crystalline  molecule,  his  paper  must  be  considered  as  pointing 
out  a  pathway  to  research  rather  than  as  leading  to  any  very 
definite  results. 

He  pictures  the  carbon  atom  as  a  bullet,  from  which  four 
rods  extend  to  the  solid  angles  of  a  tetrahedron.  Carbon, 
when  crystallized  in  the  form  of  diamond,  shows  exactly  the 
type  of  symmetry  exhibited  by  this  carbon  atom.  In  the 
form  of  graphite,  it  crystallizes  in  the  hexagonal  system  and 
apparently  shows  rhornbohedral  hemihedrism.  Now  if  two 
carbon  atoms  are  so  arranged  that  two  valencies,  or  rods,  one 
from  each  atom,  fall  in  the  same  straight  line,  and  the  three 
remaining  rods  from  each  of  the  two  atoms,  instead  of  being 
directly  opposite  one  another,  are  arranged  alternately,  as  in 
the  figure, 

1  Zeit.  f.  Krystallographie,  xx.,  76,  435. 


VARIATIONS  IN  OPTICAL  ACTIVITY.  151 


we  have  the  symmetry  of  the  rhombohedral  hemihedrism  of 
the  hexagonal  system.  This  desired  condition  of  affairs  is 
brought  about  by  the  assumption  that  the  crystalline  molecule 
in  graphite  consists  of  two  carbon  atoms,  —  an  unproved  and, 
as  Fock  himself  states,  an  improbable  assumption,  but  with  a 
larger  number  of  atoms  it  would  not  be  (difficult  to  construct 
the  symmetry  of  the  hexagonal  system  in  a  similar  way.  On 
a  similar  basis  Fock  attempts  to  explain  the  crystalline  form 
of  calcite  on  the  assumption  that  the  crystalline  molecule  is 
Ca2(C03)2,  and  to  show  some  reasons  for  the  similarity  in 
crystalline  form  of  CaC03  and  NaN03. 

In  other  compounds  of  carbon  also,  a  possible  correlation 
between  the  crystalline  form  and  the  geometrical  form  of  the 
chemical  molecule  is  shown,  but  the  paper  is  interesting,  not 
so  much  for  the  actual  results  achieved,  as  being  one  of  the 
pioneer  efforts  to  link  together  stereo-chemistry  and  crystal- 
lography. "No  one  can  study  the  former  subject  without  feel- 
ing that  it  may  be  destined  to  give  much  aid  to  the  latter. 
Already  the  analogy  in  behavior  between  crystals  and  organic 
molecules  in  regard  to  polarized  light  has  led  to  the  as- 
sumption of  geometrical  molecules,  —  the  first  step  in  stereo- 
chemistry, —  and  now  the  effort  has  been  started  to  explain 
the  building  up  of  crystals  from  these  geometrical  molecules. 
If  the  crystal  units  of  a  number  of  different  substances  could 
be  determined,  the  subject  would  be  much  simplified,  and  the 
bearings  of  isomorphism  and  dimorphism  might  then  be  dis- 
cussed more  intelligently. 


152        THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 

It  must  be  granted  that  although  there  are  certain  relations 
between  stereo-chemistry  and  crystallography  which  naturally 
suggest  themselves,  they  can  be  but  vaguely  traced  at  present, 
but  they  give  promise  of  becoming  plainer  in  the  future,  and 
thus  furnishing  another  proof  of  the  interdependence  of  all 
science ;  and  there  seems  every  reason  to  hope  that  many 
other  interesting  developments  may  be  brought  out  by  a 
further  study  of  these  two  sciences  in  their  relations  to  each 
other. 


VI. 


DEDUCTIONS  AND  SPECULATIONS  CONCEENING 
THE  NATUEE  OF  ATOMS  AND  VALENCE, 
WHICH  HAVE  GEOWN  OUT  OP  THE  STUDY  OF 
STEEEO-CHEMISTEY. 

THE  study  of  geometrical  isomerism  cannot  fail  to  stimulate 
a  new  interest  in  the  old  unanswered  questions  as  to  the 
nature  and  form  of  an  atom,  and  the  real  significance  or  nature 
of  that  property  of  the  atom  which  we  call  valence,  and  the 
present  work  would  be  incomplete  without  some  review  and 
discussion  of  the  various  ideas  and  hypotheses  which  have 
grown  out  of  stereo-chemical  studies.  Stereo-chemistry  has 
given  us  a  much  more  definite  idea  of  organic  molecules  than 
we  have  ever  had  before ;  it  has  entered  so  far  into  their  con- 
stitution as  to  represent,  in  addition  to  the  atomic  linking,  the 
relative  arrangement  of  the  atoms  in  space,  and  also  to  de- 
scribe certain  definite  motions  which  may  be  taking  place 
within  the  molecule ;  therefore  we  have  some  right  to  suppose 
that  it  may  be  able  to  throw  new  light  on  the  subjects  of 
atoms  and  valence.  There  are  many  questions  which  have 
long  puzzled  the  most  profound  thinkers,  and  it  has  almost 
seemed  as  if  they  must  forever  remain  unanswered,  from  the 
nature  of  the  case,  but  now  that  stereo-chemistry  has  pene- 
trated so  deeply  into  the  invisible  realm  of  the  infinitesimally 
small,  we  begin  to  ask  if  it  may  not  go  still  farther  and  throw 
some  light  on  these  old  hard  questions. 


154        THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 

In  what  follows  we  shall  accept  the  generally  received 
results  of  stereo-chemistry,  and  simply  ask  what  conclusions 
can  be  drawn  from  them  in  regard  to  atoms  and  valence. 
Stereo-chemistry  has  explained  so  many  cases  of  isomerism, 
and  is  in  harmony  with  so  many  known  facts  in  organic  chem- 
istry, that  it  can  no  longer  be  regarded  as  a  doubtful  hypoth- 
esis ;  whatever  changes  in  form  or  symbolism  it  may  undergo 
in  the  future,  we  cannot  but  feel  that  it  has  sufficient  under- 
lying truth,  so  that  its  bearings  on  the  subject  of  the  unseen 
atoms  are  worthy  of  serious  attention.  On  many  of  the  ques- 
tions which  have  long  puzzled  us,  stereo-chemistry  is  silent,  to 
a  few  it  gives  apparently  definite  answers,  and  in  regard  to 
others  it  offers  suggestions  which  may,  unfortunately,  be 
variously  interpreted. 

Some  of  the  questions  which  have  long  furnished  interest- 
ing subjects  for  discussion  are  the  following :  Are  the 
chemical  atoms  really  aro/xot  in  the  sense  of  being  from  their 
nature  indivisible,  or  are  they  simply  the  limit  reached  by 
chemical  means,  just  as  by  mechanical  means  a  tiny  fragment 
may  be  obtained  which  we  can  no  further  subdivide,  though 
itself  made  up  of  parts  and  therefore  conceivably  capable  of 
still  further  subdivision  ?  Are  the  atoms  of  the  different  ele- 
ments to  be  considered  as  so  many  separate  and  distinct  forms 
of  matter,  or  should  they  rather  be  regarded  as  made  up  from 
one  primordial  element,  differing  from  one  another  either  in 
the  amount  of  condensation  of  the  matter,  or  in  the  kinds  of 
motion  with  which  they  are  impressed,  or  differing  simul- 
taneously in  both  ways  ?  Shall  we  look  upon  the  atoms  of 
all  elements  as  material  points  from  which  forces  proceed,  and 
therefore  disregard  shape  and  size,  or  shall  we  consider  them 
as  structures  having  a  finite  expansion  in  space,  and  therefore 
capable  of  existing  in  different  geometrical  forms  ? 


DEDUCTIONS  AND   SPECULATIONS.  155 

Along  with  these  questions  must  also  go  queries  concerning 
the  nature  of  valence.  Is  it  an  inherent  property  of  the  atom, 
or  is  it  first  called  into  existence  by  the  approach  of  other 
atoms  ?  Has  it  definite  location  in  the  atom,  is  it  exerted  in 
certain  definite  directions,  is  it  to  be  considered  as  originally 
divided  into  parts  in  the  atom  ;  or  is  it  more  of  the  nature  of 
other  attractive  forces,  an  undivided  whole,  until  the  near 
approach  of  other  atoms  causes  it  to  be  divided  among  them  ? 
In  what  does  the  difference  of  valence  in  different  elements 
consist  ?  Does  it  correspond  to  the  difference  in  different 
magnets,  a  difference  in  the  amount  of  attractive  force ;  or  to 
a  difference  in  the  motions  of  the  atoms,  perhaps  a  different 
number  of  vibrations  in  a  unit  of  time ;  or  to  a  difference  in 
the  number  of  certain  particular  parts  of  the  atom  which  we 
may  call  valence-places  ?  What  do  we  mean  by  double  and 
triple  linking  between  two  carbon  atoms,  and  what  conceptions 
of  valence  can  explain  the  fact  that  doubly  linked  carbon 
atoms  are  not  held  together  with  twice  the  strength  of  two 
singly  linked  atoms,  and  triply  linked  with  three  times  that 
strength  ? 

In  taking  up  this  portion  of  the  subject,  it  cannot  be  devel- 
oped from  a  historical  standpoint,  since  there  has  been  no 
gradual  growth  from  one  theory  to  another  until  one  is  finally 
left  in  triumphant  possession  of  the  field.  We  can  only  take 
a  brief  review  of  some  of  the  most  important  deductions  and 
speculations  which  have  grown  up  out  of  stereo-chemical 
theories,  and  discuss  and  classify  them  as  far  as  possible. 

The  three  attributes  of  the  atom  which  enter  most  into 
stereo-chemical  discussion  are  its  motions,  its  valence,  and  its 
form ;  and  these  will  be  considered,  as  far  as  may  be,  in  the 
order  here  given,  though  it  is  manifestly  impossible  to  sepa- 
rate the  three  subjects  entirely  from  one-  another.  The  illus- 


156    THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

trations  and  applications  of  the  theories  must  necessarily  be 
limited  almost  entirely  to  the  carbon  atom,  though  some  gen- 
eralizations can  be  drawn  which  will  apply  to  all  atoms. 

The  most  intimate  study  of  molecules  has  been  thus  far 
almost  entirely  restricted  to  the  investigation  of  the  relative 
positions  of  the  atoms ;  and  although  motion  is  assumed,  both 
of  the  molecule  as  a  whole  and  of  the  component  atoms,  no 
very  definite  hypotheses  as  to  4;he  latter  motions  have  been 
presented.  It  is  very  probable  that  the  next  advances  will  be 
made  in  this  direction.  The  rapid  advances  in  physics,  in  con- 
nection with  the  knowledge  of  molecular  motions,  has  led  to 
the  belief  that  we  can  get  no  satisfactory  and  adequate  idea  of 
chemical  phenomena  until  some  account  is  made  of  atomic 
motions,  as  well  as  of  atomic  arrangements.  More  than  once 
already  has  attention  been  called  to  the  necessity  of  advance 
in  this  direction ;  and  though  the  task  seems  difficult,  to  one 
who  has  followed  the  progress  made  in  our  knowledge  of  the 
internal  constitution  of  molecules  during  the  last  twenty 
years,  nothing  seems  impossible. 

Berthelot,1  as  early  as  1875,  stated  it  as  his  opinion  that 
any  explanation  of  the  different  kinds  of  optical  bodies,  based 
on  the  arrangement  of  the  atoms  in  the  molecule,  was  insuffi- 
cient, but  that  it  was  necessary  also  to  take  into  account  their 
vibratory  and  rotatory  motions.  Thus,  he  suggests  that  opti- 
cally active  and  inactive  bodies  may  be  explained  in  the  fol- 
lowing way :  In  a  molecular  system  we  may  conceive  that 
the  atoms  all  vibrate  in  the  same  plane,  giving  the  inactive 
body ;  or  they  may  vibrate  in  another  plane,  inclined  symmet- 
rically to  the  right  or  left,  in  relation  to  the  fundamental 
atoms,  thus  giving  the  right  and  left  bodies ;  or  we  may  con- 
ceive, again,  two  symmetrical  systems  touching  one  another  in 

i  Bull.  Soc,  Chim.  [2]  23,  338. 


DEDUCTIONS  AND  SPECULATIONS.        157 

such  a  way  as  to  give  a  mean  state  of  movement  analogous  to 
that  of  the  inactive  body,  and  this  is  the  neutral  body. 

Quite  recently  Molinari1  has  again  opened  up  the  subject 
of  vibratory  motions  in  an  article  under  the  title,  "Stereo- 
chemistry, or  Moto-cheniistry  ?  "  He  offers  some  objections 
to  the  present  stereo-chemical  theories,  strenuously  opposes 
the  hypothesis  of  limited  rotation  proposed  by  Auwers  and 
Meyer,  and  objects  to  the  supposition  that  rotation  must  be 
stopped  in  doubly  linked  carbon  compounds.  In  regard  to  his 
first  objection,  —  since  the  idea  of  limited  rotation  can  hardly 
be  said  to  rank  as  a  fully  developed  stereo-chemical  theory, 
having  only  a  very  few  facts,  and  those  somewhat  uncertain, 
which  its  most  earnest  advocates  can  bring  forward  in  proof  of 
its  necessity,  —  it  would  seem  that  it  should  hardly  be  used  as 
an  argument  against  the  main  stereo-chemical  theories. 

Molinari  also  argues  that  unless  a  bond  is  something  hard 
and  stiff,  which  does  not  appear  probable,  he  cannot  under- 
stand why  there  should  be  motion  when  two  atoms  are  joined 
with  one  such  bond,  and  an  unnatural  state  of  rest,  such  as  is 
met  nowhere  else  in  the  universe,  when  the  two  atoms  are 
doubly  linked.  But  van't  HofFs  hypothesis  does  not  state 
that  all  motion  is  necessarily  stopped  in  such  compounds,  — 
only  that  one  particular  kind  of  motion  must  cease.  Rotation 
around  the  axis  joining  the  two  carbon  atoms  is  now  impos- 
sible, but  oscillations  to  and  fro  around  a  fixed  mean  position 
may  still  take  place. 

An  objection  against  the  stereo-chemistry  of  to-day,  how- 
ever, which  cannot  be  gainsaid,  is  that  there  are  still  a  number 
of  facts  of  organic  chemistry  which  cannot  be  explained  by 
our  present  theories.  The  facts  of  tautomerism,  for  example, 
show  that  any  theory  with  relation  to  the  position  of  the 
i  J.  f.  pr.  Chem.  48,  113. 


158        THE  DEVELOPMENT  OF   STEREO-CHEMISTRY. 

atoms  is  insufficient  to  explain  all  phenomena  ;  and  it  would 
seem  that  a  theory  of  motion  must  be  introduced  to  account 
for  such  bodies.  Molinari  also  mentions  several  instances  of 
isoinerism  still  unexplained,  —  some  of  these,  however,  being 
rather  doubtful  cases,  and  asks  if  the  different  isomeric  forms 
could  not  be  equally  well  explained  by  the  supposition  of  dif- 
ferences in  the  motions  of  the  atoms.  His  ideas  are  interest- 
ing, partly  because  suggesting  a  dynamical  basis  for  valence; 
but  it  is  certainly  questionable  whether  any  theory  of  motion, 
taken  by  itself,  would  prove  any  more  satisfactory  than  a  the- 
ory of  position  alone.  It  is  probable  that  before  every  pos- 
sible case  of  isomerism  is  explained,  both  the  arrangement  and 
motions  of  the  atoms  will  have  to  be  taken  into  account. 

To  illustrate,  briefly,  Molinari's  theory,  since  the  valence  of 
carbon  is  IV,  we  may  assume  that  it  makes  four  vibrations  in 
one  unit  of  time.  In 


C  may  either  strike  a  once,  then  a',  then  b  and  b'}  and  repeat 
this  motion  periodically  ;  or  it  may  strike  a  first,  then  b,  then 
of  and  bf  ;  and  if  a  and  a',  and  b  and  &',  represent  the  same 
elements,  these  are  the  only  two  kinds  of  impacts  possible. 
The  question  might  here  be  raised,  if  isomerism  depends  on 
the  kind  of  impacts,  —  and  there  may  be  two  such  impacts  for 
a  body  like  CH2C12,  —  why  may  not  that  body  exist  in  two 
different  isomeric  forms  ? 

According  to  Molinari,  double  linking  between  two  carbon 
atoms  might  indicate  two  consecutive  and  inseparable  impacts 
in  unit  of  time.  The  theory  is  ingeniously  worked  out  for 


DEDUCTIONS  AND   SPECULATIONS.  159 

benzole  and  other  'compounds,  but  need  not  be  discussed  in 
detail  here,  since  it  is  a  practical  abandonment  of  stereo- 
chemistry for  a  much  less  complete  and  satisfactory  explana- 
tion of  the  facts  observed.  In  connection  with  these  studies 
it  is  only  interesting  to  note  that  the  deeper  investigation  of 
the  molecules  undertaken  in  stereo-chemistry  has  led  to  the 
stronger  feeling  that  the  motions  of  the  atoms  are  also  of  the 
greatest  importance. 

A  portion  of  BischofFs  work  has  already  been  briefly  alluded 
to,  but  his  "  dynamic  hypothesis  "  may  appropriately  be  taken 
into  consideration  here  as  indicating  the  tendency  to  take  the 
motions  of  the  atoms  into  account  in  explaining  chemical  phe- 
nomena. It  will  be  remembered  that  he  thinks  the  position 
of  stable  equilibrium  is  best  represented  by  assuming  that  the 
radicals  attached  to  two  singly  linked  carbon  atoms  are  as  far 
apart  as  possible.  Thus,  the  molecule  of  ethane,  viewed  from 
above,  would  be  represented  by  him  as  follows :  — 


H 

A 


H 


V 


H 

In  the  above  formula,  since  the  three  groups  attached  to  one 
carbon  atom  are  all  alike,  the  figure  formed  by  joining  these 
groups  by  straight  lines  is  an  equilateral  triangle;  if  the 
groups  were  CH3,  CH8,  and  COOH,  it  would  be  an  isosceles 
triangle ;  and  if  H,  CH3,  and  COOH,  the  triangle  would  be 
scalene.  Whether  the  planes  represented  by  the  two  triangles 
are  parallel,  or  inclined  to  one  another,  does  not  enter  into  the 

discussion. 

/-^eSE    LIBRAE 

f       *  OF  THE 

(UNIVERSITY) 


160        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

Erom  the  dynamical  standpoint,  that  configuration  is  most 
favorable  for  stability  in  which  the  individual  components 
of  the  molecule  are  reciprocally  hindered  the  least  in  their 
oscillations. 

If,  in  the  above  formula,  two  adjacent  hydrogen  atoms  are 
replaced  by  carboxyls,  the  result  of  raising  the  temperature 
will  naturally  be  to  increase  the  oscillations  of  these  atoms, 
until  it  may  happen  that  they  approach  near  enough  to  be 
within  each  other's  sphere  of  action,  when  water  splits  off. 
This  Bischoff  calls  a  "  collision."  Now,  it  has  been  found  that 
the  addition  of  methyl  groups  increases  the  ease  of  dehydra- 
tion, and  Bischoff  explains  this  by  assuming  a  repulsion  be- 
tween methyl  and  carboxyl.  Succinic  acid,  according  to  him, 
is  represented  by  the  formula, 


Hr  —    -    —  7  COOH 


H 

in  which  the  carboxyls  keep  as  far  apart  as  possible,  on  ac- 
count of  their  weak  repulsion  for  each  other.  By  the  intro- 
duction of  methyl  for  one  of  the  hydrogen  atoms,  the  stronger 
repulsion  between  methyl  and  carboxyl  drives  the  carboxyls 
as  far  as  possible  from  the  methyl,  and  therefore  nearer  each 
other,  making  it  easier  to  form  the  anhydride,  as  is  found  in 
fact  to  be  the  case.  This  ease  of  dehydration  increases  as  the 
number  of  methyl  groups  increases,  tetram ethyl  succinic  acid 
forming  its  anhydride  most  easily  of  any. 

The  repulsion  between  alkyl  and  carboxyl  groups  finds  a 


DEDUCTIONS  AND  SPECULATIONS.  161 

farther  support  in  the  non-existence  of  a  methylmaleic  acid  of 
formula 

CH3         COOH 
\  / 
C 

II 

C 

/  \ 
COOH         CH8 

which,  according  to  Wislicenus's  view,  should  be  stable, 
whereas  all  reactions  in  which  the  formation  of  this  body 
might  be  expected,  give,  instead,  the  anhydride  of  its  geomet- 
rical isonier,  pyrocinchonic  acid, 

CH8         COOH 

\  / 

C 

II 

C 

/  \ 
CH8         COOH 

The  same  is  true  in  regard  to  ethylmaleic,  or  xeronic  acid. 
Here  the  space  taken  up  by  the  groups  in  their  oscillations 
seeras  to  be  a  matter  of  some  consequence.  Thus,  the  superior 
stability  of 

CH8         COOH 
\  / 
C 

II 

C 

/  \ 
CH8         COOH 

over  its  geometrical  isomer  is  explained  by  Bischoff  on  the 
assumption  of  repulsion  between  methyl  and  carboxyl ;  but  in 

11 


162         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

this  molecule  the  methyl  groups  take  up  so  much  more  room 
than  would  be  occupied  by  hydrogen  atoms  that  there  is  a 
crowding  together  of  atoms  inside  the  molecule,  a  limiting  of 
vibrations,  so  that  collisions  take  place  more  readily,  resulting 
in  the  splitting  off  of  water. 

A  collision  between  two  carboxyl  groups  leads  to  a  splitting 
off  of  water ;  when  methyl  collides  with  carboxyl,  there  is  no 
chemical  interaction,  but  the  carboxyl  is  driven  back  along 
the  path  through  which  it  came,  and  the  oscillations  are  lim- 
ited in  extent.  Bischoff 1  thinks  that  through  the  limitation 
of  vibrations,  caused  by  introduction  of  alkyl  groups,  certain 
systems  which  have  been  considered  as  freely  rotatable  around 
an  axis  may  be  so  only  up  to  a  certain  limit,  and  that,  there- 
fore, configurations  which  have  been  considered  identical  are 
not  necessarily  so;  but  configuration  symbols  may  be  produced 
for  two  bodies,  differing  in  the  same  way  as  the  ordinary  geo- 
metrical isomers,  though  perhaps  less  in  amount.  To  this 
new  kind  of  isornerisin,  based  on  the  consideration  of  the 
space  filled  by  the  radicals,  and  the  shortening  of  the  distance 
between  the  carbon  atoms  caused  by  this,  he  gives  the  name, 
"  dynamical  isomerism."  In  these  bodies  the  kinetic  energy 
of  the  individual  motions  of  the  atoms  inside  the  molecule  is 
not  sufficient  to  conquer  the  resistance  which  the  jostling  of 
the  radicals  against  each  other  opposes  to  the  rotation  of  the 
one  form  into  the  other.  If  energy  is  added,  then  the  impacts 
may  become  so  lively  that  this  resistance  is  overcome,  and  by 
increasing  the  distance  of  the  carbon  atoms  from  one  another 
by  heat,  the  transformation  of  one  form  into  another  may  be 
accomplished. 

Bischoff  has  applied  his  views  very  successfully  to  the  ex- 
planation of  the  substituted  succinic  acids,2  and  also  to  the  y 

i  Ber.  23,  623 ;  24,  1077.  2  Ber.  24, 1085. 


DEDUCTIONS   AND   SPECULATIONS.  163 

ketone  acids ; 1  but  they  do  not  seem  to  be  in  a  sufficiently 
clear  and  definite  form  to  be  of  very  general  application  at 
present.  This  "  dynamical  hypothesis  "  has  also  been  used  by 
Bischoff  to  explain  the  stability  of  certain  bodies,  such  as  the 
carbonates,  notwithstanding  the  instability  of  the  closely  re- 
lated compound,  carbonic  acid.2  According  to  this  explanation, 
the  sodium,  for  example,  will  not  take  the  exact  place  of  the 
hydrogen  atom,  for  which  it  is  substituted,  but  on  account 
of  its  greater  attraction  for  the  oxygen  it  will  draw  the  latter 
atom  nearer  to  itself,  and  farther  from  the  carbon.  Thus,  we 
may  represent  carbonic  acid  by  the  following  formula,  in  which 
there  is  not  room  for  free  oscillation,  but  a  collision  would 
occur  by  which  water  would  be  given  off :  — 


If,   however,   one    of    the    hydrogen    atoms  be   replaced  by 
sodium,  the  following  formula  results :  — 


and  for  Na2C03, 


In  these  last  two  formulas,  the  attraction  of  sodium  for 
oxygen  is  shown  to  have  drawn  the  latter  farther  from  the 
carbon,  and  thus  the  interference  seen  in  carbonic  acid  is 
•avoided. 

Another  possibility  suggested  is  that  the  introduction  of  the 
sodium  may  alter  the  angle  between  the  carbon  and  oxygen 
without  changing  the  distances  between  the  atoms.  This  pos- 
sibility is  represented  by  the  following  formulas :  — 

1  Ber.  26,  1452.  2  Ber.  23,  3414. 


164        THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 


Now,  it  is  possible  for  two  hydroxyl  groups  to  be  as  near 
together  when  joined  to  two  different  carbon  atoms  as  when 
both  are  united  to  the  same  one,  therefore  exactly  similar 
explanations  may  be  applied  to  other  bodies  as,  for  example, 
to  show  why  pyrocinchonic  acid  exists  only  in  the  form  of 
the  anhydride,  whereas  its  ethyl  salt  is  stable. 

Another  attempt  to  explain  observed  phenomena  by  refer- 
ence to  the  motions  of  atoms  is  made  by  Werner,  who  refers 
the  change  from  the  active  to  the  inactive  body,  under  the 
influence  of  heat,  to  the  oscillations  of  the  atoms  in  the  mole- 
cule. His  idea  of  the  carbon  atom  is  a  sphere  with  four 
"  valence-places."  These  valence-places  are  the  four  points 
where  lines  joining  the  centre  of  gravity  of  the  carbon  atom 
with  the  centres  of  gravity  of  each  of  the  four  radicals,  cut 
the  surface  of  the  sphere.  The  atoms  are  assumed  to  have 
certain  periodic  motions  around  the  valence-places,  and  certain 
influences  can  increase  or  decrease  these  motions.  Among  the 
simplest  forms  of  oscillation,  we  may  assume  that  the  atoms 
vibrate  in  two  planes  at  right  angles  to  each  other. 


III 


DEDUCTIONS  AND   SPECULATIONS.  165 

In  Figure  I.,  suppose  a,  b,  c,  d,  to  be  four  radicals  oscillating 
in  the  directions  shown  by  the  arrows.  Then,  if  these  motions 
be  increased  by  heat,  the  atoms  may  vibrate  so  far  as  to  reach 
the  position  shown  in  Figure  II.  From  this  position,  they 
can  pass  as  readily  into  that  shown  in  III.  as  to  return  to 
their  original  position,  therefore  the  final  result  of  heating 
such  a  configuration  as  that  shown  in  I.  would  be  to  produce 
equal  amounts  of  I.  and  III.  If,  then,  I.  represents  an  active 
body,  and  III.  its  geometrical  isomer,  the  result  of  heating 
either  would  naturally  be  to  produce  inactivity. 

The  facts  in  regard  to  the  motions  of  atoms,  as  derived 
from  a  study  of  stereo-chemistry,  are  certainly  not  very  defi- 
nite. Constant  atomic  motion  of  one  kind  or  another  is  as- 
sumed. In  the  case  of  singly  linked  carbon  atoms,  a  revolution 
of  the  two  atoms  in  opposite  directions  around  an  axis  joining 
the  two  is  supposed  possible,  but  when  the  kind  of  linkage 
changes  to  double  or  triple,  the  rotatory  motion  is  assumed  to 
cease,  so  that  in  the  resulting  molecule  the  only  motions  pos- 
sible are  those  of  the  molecule  as  a  whole,  an  oscillatory 
motion  of  the  radicals  connected  with  the  carbon,  and  vibra- 
tory motions  of  the  carbon  atoms  themselves.  Bischoff  adds 
to  this  the  idea  of  the  oscillations  of  the  atoms  in  the  mole- 
cule, producing  such  jostling  against  each  other  that  certain 
groups  are  shaken  off,  thus  accounting  for  various  decomposi- 
tions. It  is  plain  that  in  this  he  also  takes  into  account  the 
size  of  the  atoms,  and  assumes  that  they  are  not  inconsiderable 
in  relation  to  the  distance  between  them. 

It  is  very  evident  that  one  of  the  weakest  points  in  our 
knowledge  of  atomic  science  to-day  is  in  regard  to  the  motions 
of  the  atoms,  upon  which,  doubtless,  depends  the  explanation 
of  many  phenomena.  On  the  subject  of  valence,  stereo- 
chemistry leads,  perhaps,  to  more  definite  notions  than  in 


166        THE   DEVELOPMENT   OF   STEREO-CHEMISTRY. 

regard  to  atomic  motions.  It  is  almost  impossible  to  accept 
all  of  the  stereo-chemical  theories  without  getting  the  im- 
pression of  valence  as  having  a  directive  action;  unlike  the 
force  of  gravitation,  for  example,  the  force  which  draws  other 
atoms  to  carbon  seems  to  be  exerted  only  in  certain  directions, 
or  from  certain  points  of  the  atom.  Fock  has  already  been 
quoted  as  stating  that  "  One  of  the  positive  results  of  stereo- 
chemical  investigation  is  to  show  that  valencies  have  a  definite 
direction."  Some  adopt  this  view  without  any  further  expla- 
nation ;  others  attribute  it  to  the  existence  of  four  places  on 
the  carbon  atom,  which  are  qualitatively  different  from  the 
rest  of  the  atom,  and  which  are  called  valence-places.  It  is, 
therefore,  plainly  impossible  to  separate  the  subjects  of  valence 
and  form  of  the  atom  entirely  from  one  another ;  they  must 
be  considered  together. 

Because  many  cases  of  organic  isomerism  can  be  best  ex- 
plained by  the  geometrical  conception  of  a  tetrahedron  having 
the  carbon  atom  in  the  centre,  and  the  combining  atoms  or 
radicals  in  the  solid  angles,  are  we  to  assume  that  the  carbon 
atom  has  really  this  geometrical  form,  with  its  attractive  force 
concentrated  in  the  four  solid  angles,  or  is  this  to  be  taken 
merely  as  a  useful  symbol  ?  And  if  the  latter,  what  is  there 
in  the  actual  nature  of  the  atom  which  could  correspond  to 
such  a  symbol  ?  Since  van't  Hoff  has  been  the  leader  in  the 
use  of  this  symbol,  it  becomes  interesting  to  find  out  what  his 
own  ideas  were  in  regard  to  the  nature  of  the  carbon  atom. 
In  his  earliest  work,  although  he  does  not  express  himself 
definitely  on  the  subject,  he  tacitly  assumes  that  the  atom  is  a 
material  point.  We  have  no  difficulty  in  reconciling  the  tetra- 
hedral  symbol  with  this  view,  so  long  as  we  confine  ourselves 
to  a  single  carbon  atom.  We  suppose  this  material  point  to 
be  endowed  with  a  certain  amount  of  attractive  force,  such 


DEDUCTIONS  AND    SPECULATIONS.  167 

that  it  can  hold  four  hydrogen  atoms.  These  four  atoms  will 
naturally  arrange  themselves  regularly  around  the  carbon,  and 
may,  therefore,  determine  the  solid  angles  of  a  regular  tetra- 
hedron. If  the  four  combining  atoms  are  unlike,  the  general 
tetrahedral  form  will  still  be  kept,  though  it  may  become 
irregular,  owing  to  an  unequal  distribution  of  the  attractive 
force  between  the  four  atoms  or  radicals,  or  to  the  action  of 
these  on  each  other. 

In  the  case  of  singly  linked  carbon  atoms,  the  possibility  of 
rotation  around  an  axis  must  be  assumed,  and  it  becomes 
questionable  whether  this  is  reconcilable  with  the  conception 
of  the  atom  as  a  material  point,  since,  in  the  words  of  Clerk 
Maxwell,  "Even  an  atom,  when  we  consider  it  capable  of 
rotation,  must  consist  of  many  material  particles."  Even  more 
serious  difficulties  confront  us  when  we  try  to  reconcile  this 
view  with  the  conceptions  of  doubly  and  triply  linked  atoms. 
If  the  atoms  are  material  points,  the  force  holding  them  to- 
gether must  act  directly  in  a  straight  line  between  the  two 
atoms,  whether  they  are  singly  or  doubly  linked.  Then  why 
should  rotation  of  the  atoms  in  reverse  directions  be  impos- 
sible in  the  latter  case  ?  This  cessation  of  rotation  would 
require  for  its  explanation  the  idea  of  the  attractive  forces 
between  these  two  points  acting  in  two  different  directions, 
and  cutting  each  other  at  an  angle,  which  is  manifestly 
inconceivable. 

If  we  accept  van't  HofPs  explanation  of  the  isomerism  in 
the  ethylenic  compounds,  we  are  thus  compelled  to  reject  the 
idea  that  the  carbon  atom  is  a  material  point,  or  even  a  homo- 
geneous sphere,  sending  out  force  in  all  directions.  We  are 
apparently  forced  to  the  conclusion  that  there  are  definite 
points  in  the  atom  from  which  the  attraction  proceeds,  or  that 
for  some  reason  it  proceeds  only  in  definite  directions,  or  that 


168         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

it  depends  in  some  entirely  unknown  way  upon  the  kind  or 
rate  of  motion  of  certain  parts  of  the  atom,  supposing  it  to  be 
made  up  of  parts. 

Lessen J  was  one  of  the  first  to  point  out  the  above-mentioned 
discrepancy  in  van't  Hoff 's  work,  and  stated  it  as  his  opinion 
that  atoms  are  not  material  points,  but  that  parts  are  to  be 
distinguished,  from  which  the  attraction  for  other  atoms  pro- 
ceeds. He  criticised  the  workers  in  stereo-chemistry  in  gen- 
eral on  the  ground  that  until  more  was  known  concerning  the 
shape,  size,  and  nature  of  atoms,  we  were  not  in  a  position  to 
decide  as  to  their  arrangement  in  space. 

This  drew  forth  an  answer  from  Wislicenus,2  in  which  is  set 
forth,  briefly,  his  own  conceptions  of  atoms.  He  urges  that 
the  question  of  atomic  arrangement  in  space  is  capable  of 
experimental  proof,  and  the  study  of  this  is  the  only  way  he 
knows  of  to  lead  up  to  the  answer  of  questions  concerning  the 
shape  and  size  of  atoms.  He  agrees  with  Lossen  that  the 
results  of  stereo-chemical  investigation  exclude  the  supposition 
that  the  atoms  are  material  points,  and  thinks  that  what  we 
call  an  atom  is  more  probably  a  complex  body.  Though  he 
states  that  no  positive  answer  has  yet  been  given  by  the  infer- 
ences from  stereo-chemistry,  he  considers  it  more  probable  that 
the  atoms  are  structures  occupying  space,  and  made  up  of 
atoms  of  some  primordial  element,  than  that  they  are  points 
bearing  energy ;  that  they  are,  therefore,  comparable  to  com- 
pound radicals,  and  that,  like  the  latter,  their  units  of  affinity 
have  position  in  certain  parts  of  them,  from  which  they  work. 
He  considers  it  not  impossible  that  the  carbon  atom  is  a  struc- 
ture which,  in  its  form,  approaches  more  or  less  nearly  to  a 
regular  tetrahedron,  perhaps  very  nearly ;  and  that  the  cause 
of  that  force  which  reaches  its  outward  manifestation  in  the 

i  Ber.  20,  3306.  *  Ber.  21,  581. 


DEDUCTIONS  AND   SPECULATIONS.  169 

form  of  "  units  of  affinity  "  may  be  concentrated  in  the  solid 
angles  of  the  tetrahedral  figure,  similarly,  and  on  analogous 
grounds,  to  the  electrical  action  of  an  electrically  charged 
metallic  tetrahedron. 

Since  this  is  applying  the  results  of  stereo-chemical  investi- 
gation to  the  shape  of  the  carbon  atom  with  the  most  literal 
exactness,  there  can,  of  course,  be  no  collision  between  the 
two. 

The  old  question  as  to  whether  that  which  resists  all  of  our 
efforts  at  subdivision,  so  that  we  call  it  an  atom,  is  to  be  con- 
sidered a  simple  unit,  from  its  very  nature  indivisible  or  not, 
is  thus  answered  in  the  negative  by  Wislicenus,  who  considers 
that  it  is  more  probably  a  structure,  and  therefore  capable  of 
existing  in  a  definite  geometrical  form.  Now,  whether  an 
atom  is  really  made  up  of  parts  or  not,  it  is  certain  that 
it  is  so  constituted,  or  so  endowed  with  a  particular  kind  of 
motion,  that  it  resists  further  subdivision  with  the  greatest 
tenacity,  and  this  seems  more  compatible  with  a  spherical 
shape  than  witl\  such  a  form  as  the  tetrahedral.  Karsten 
says  that  a  crystal  is  the  result  of  cohesion  working  unequally 
in  different  directions ;  but  if  the  cohesion  works  equally  in 
an  infinite  number  of  directions,  the  result  is  a  sphere.  The 
idea  of  the  tetrahedral  atom  suggests  this  difference  of  cohe- 
sion in  different  directions,  which  might  be  expected  to  give 
the  atom  a  tendency  to  break  in  certain  directions,  and  there- 
fore destroy  its  stability.  The  figure  of  an  atom  with  sharp 
corners  also  suggests  to  the  eye  the  possible  breaking  off  of 
these  corners ;  i.  e.,  the  subdivision  of  the  atom,  on  slight 
provocation. 

Another  fact  which  would  seem  to  be  opposed  to  Wislice- 
nus's  suggestion,  which,  it  must  be  remembered,  he  brought 
forward  only  very  tentatively,  is  that,  according  to  this,  two 


170         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

triply  linked  carbon  atoms  should  present  the  most  stable  con- 
figuration possible.  Two  tetrahedrons  held  tightly  together 
by  the  forces  concentrated  at  three  solid  angles  of  each,  no 
rotation  possible,  where  is  there  a  point  of  attack  ?  How, 
then,  can  the  comparative  instability  of  such  bodies  be  ex- 
plained, and  the  ease  with  which  addition  compounds  are 
formed  ?  In  fact,  it  is  difficult,  with  this  conception  of  an 
atom,  to  see  how  it  is  possible  for  a  double  linkage  to  be  any- 
thing but  exactly  twice  as  strong  as  a  single,  and  a  triple 
union  to  possess  three  times  that  strength.  This  is  an  objec- 
tion which  has  often  been  brought  against  "bonds  of  valence," 
however  used.  It  is  urged  that  if  a  "bond"  is  a  unit  of 
attractive  force,  then  a  double  bond  must  necessarily  have 
twice  the  strength  of  one,  etc.  Certain  conceptions  of  valence 
have,  however,  grown  out  of  stereo-chemical  discussions,  which 
show  this  not  to  be  a  necessity,  and  in  so  far  agree  with  the 
results  of  experience ;  and  some  investigators  have  gone  so  far 
as  to  calculate,  from  the  assumptions  made,  a  mathematical 
value  for  the  relative  strength  of  single,  double,  and  triple 
linkage. 

The  chemists  who  have  given  the  most  attention  to  the  sub- 
jects of  double  and  triple  bonds,  and  the  change  from  one  form 
of  linkage  to  another,  are  von  Baeyer  and  Naumann. 

Von  Baeyer's  strain  theory  has  already  been  referred  to  in 
connection  with  ring  formations.  This  first  appeared  in  1885,1 
in  connection  with  his  study  of  the  polyacetylene  derivatives. 
He  argues  that  the  explosibility  of  these  compounds  must 
depend  upon  the  setting  free  of  heat,  and  suggests  that  the 
energy  thus  set  free  may  have  been  held  originally  in  the 
molecule  in  the  form  of  a  "  strain,"  produced  by  the  bending 
of  the  axes  of  valence  from  their  normal  position.  He  assumes 

i  Ber.  18,  2269. 


DEDUCTIONS  AND  SPECULATIONS.        171 

that  the  carbon  atom  is  provided  with  four  such  axes,  extend- 
ing in  the  directions  of  the  angles  of  a  tetrahedron  as  their 
normal  position.  In  passing  from  single  to  double  linkage, 
two  of  these  axes  must  be  bent  from  their  normal  position 
until  they  are  parallel  ;  i.  e.,  each  one  must  be  turned  through 
an  angle  of  54°  44'.  In  passing  from  single  to  triple  linkage, 
three  axes  must  be  made  parallel  ;  i.  e.,  each  one  turned 
through  an  angle  of  70°  32'.  The  greater  the  deviation  from 
the  normal  position,  the  greater  is  the  strain,  and  consequently 
the  less  the  stability  of  the  resulting  body.  He  points  out  the 
correspondence  between  these  ideas  and  the  results  of  Thorn- 
sen's  thermo-chemical  investigations,  but  also  mentions  one 
fact  which  seems  to  be  out  of  harmony  with  his  theory.  This 
is  that  hydrogen  easily  changes  diacetylene  carbonic  acid  to 
propargylic  acid,  according  to  the  following  equation  :  — 


HC^C-COOH 

In  this  reaction,  the  single  linkage  is  shown  to  be  attacked 
before  the  triple.  The  weak  point  of  von  Baeyer's  explana- 
tion seems  to  be  the  difficulty  of  conceiving  of  anything  actu- 
ally existing  in  the  molecule  corresponding  to  such  elastic, 
flexible  wires  as  he  pictures  in  his  theory,  —  the  only  possible 
explanation  of  which  has  been  advanced  by  Wunderlich,  whose 
work  will  be  reviewed  later. 

Naumann  assumes  that  the  attraction  between  two  carbon 
atoms  comes  to  its  greatest  force  if  the  direction  of  the  attrac- 
tion coincides  with  the  line  joining  the  centres  of  gravity  of 
the  two  atoms.  If  the  direction  of  attraction  deviates  from 
the  cent  re  -of  -gravity  line,  then  the  attraction  comes  into  effect 
only  in  that  component  of  the  force  which  lies  in  the  above- 


172         THE  DEVELOPMENT   OF   STEREO-CHEMISTRY. 

named  direction.  For  example,  in  the  case  of  singly  linked 
atoms,  the  full  attraction  comes  into  force ;  not  so,  however, 
with  doubly  linked  atoms.  For,  representing  the  centres  of 
gravity  of  two  carbon  atoms,  doubly  linked,  by  S  and  Sj 


the  attraction,  instead  of  working  directly  from  S  to  S^  is 
directed  from  S  and  Sj  to  E  and  Ej,  and  only  that  component 
which  works  in  the  direction  S  Sx  can  produce  any  effect,  since 
the  other  components  will  be  equal  and  opposite  in  direction, 
and  therefore  neutralize  each  other. 

Starting  with  these  premises,  and  letting  S  equal  the  centre 
of  gravity  of  the  carbon  atom  considered  as  tetrahedral,  E  the 
angle  of  the  regular  tetrahedron,  K  the  middle  point  of  an 
edge,  and  F  the  middle  point  of  a  face,  it  becomes  an  easy 
matter  to  find  the  relative  strengths  of  single,  double,  and 
triple  linkage.  The  full  strength  of  the  attractive  force,  rep- 
resented by  S  E,  is  put  equal  to  1.  In  double  linkage,  as  shown 
in  the  above  figure,  only  the  component  S  K  is  taken  into  ac- 
count. Knowing  S  E  —  1,  by  the  solution  of  triangles,  S  K  = 
.5774,  but  since  there  are  two  such  bonds,  the  full  force  of  the 
attraction  is  represented  by  2  X  .5774  =  1.1548.  Similarly  in 
triple  linkage  the  force  of  each  bond  is  represented  by  S  F,  and 
this  is  found  to  equal  .3333,  and  the  total  force  of  the  three 
bonds  will  be  3  X  .3333  =  1. 


DEDUCTIONS  AND  SPECULATIONS.       173 

From  the  above  results  it  would  follow  that  the  same  force 
must  be  required  to  separate  two  carbon  atoms  when  they  are 
singly  as  when  triply  bound,  but  that  a  greater  force  is  neces- 
sary if  they  are  doubly  linked;  also,  that  in  passing  from 
triple  to  double  linkage,  each  bond,  still  occupied  in  holding 
together  the  carbon  atoms,  gains  an  amount  of  energy  which 
may  be  represented  by  the  number  .2441,  and  in  passing  from 
double  to  single  linkage  the  gain  in  energy  is  considerably 
greater,  being  represented  by  the  number  .4226 ;  or  this  may 
be  stated,  that  it  requires  a  greater  force  to  change  from 
double  to  single  linkage  than  from  triple  to  double.  The  re- 
verse changes  can  take  place  only  in  consequence  of  strong 
deviation  of  the  atoms  from  their  mean  position  of  equilib- 
rium. By  heat,  for  example,  the  oscillations  of  two  singly 
linked  carbon  atoms  may  be  so  increased  that  they  assume 
double  linkage,  etc. 

Naumann  carries  his  speculations  a  step  further,  and  says 
that  these  same  considerations  may  be  applied  to  other  poly- 
valent elements,  if  we  assume  that  the  distance  between  the 
points  of  attack  of  two  valence  units  of  other  polyvalent 
atoms  are  equal  to  those  of  carbon,  and  that  the  centres  of 
gravity  of  the  atoms  have  positions  corresponding  as  nearly 
as  possible  to  that  of  the  carbon.  Thus  he  would  represent 
nitrogen,  and  other  triad  elements,  by  an  obtuse  pyramid, 
whose  base  equals  one  face  of  a  regular  tetrahedron ;  bivalent 
elements,  by  an  isosceles  triangle,  whose  base  is  the  tetra- 
hedral  edge  of  a  carbon  atom,  and  whose  height  equals  the 
distance  from  the  centre  of  gravity  of  the  carbon  tetrahedron 
to  the  middle  point  of  an  edge.  Interesting  as  these  specula- 
tions may  be,  however,  it  seems  useless  to  spend  much  time 
upon  them  until  more  reasons  appear  for  the  assumptions  upon 
which  they  are  based. 


174        THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 

It  is  easy  to  see  from  these  considerations  that  Naumann's 
idea  of  the  carbon  atom  is  not  that  of  a  material  point,  but 
rather  of  a  structure  having  four  parts  corresponding  to  the 
solid  angles  of  a  regular  tetrahedron,  which  differ  in  proper- 
ties from  the  rest  of  the  atom,  and  may  be  called  valence- 
places.  It  does  not  appear,  however,  why  all  of  the  attractive 
force  of  the  atom  should  be  directed  from  the  centre  of  gravity 
of  the  atom  toward  these  valence-places. 

One  of  the  most  valuable  and  interesting  discussions  of  the 
carbon  atom  which  has  ever  been  published  was  made  by  Dr. 
Aemilius  Wunderlich  in  1886.1  The  assumptions  which  he 
makes  in  regard  to  atoms  are  as  follows  :  — 

(1)  The  atom  is  a  finite  quantity  of  matter  occupying  a 
finite  quantity  of  space. 

(2)  At  any  given  time  the  atom  must  possess  a  particular 
form.     Since  any  deep-seated,  constantly  recurring  change  of 
form  would  tend  to  divide  the  atom,  it  cannot  be  far  from  the 
truth  to  say  that  the  atom  has  &  fixed  form. 

(3)  On  an  atom  of  valence  n,  there  are  n  "  valence-places," 
characterized  by  the  fact  that  the  atom  is  satisfied  when  each 
of  these  n  places  is  separated  from  a  similarly  characterized 
place  on  another  atom  of  the  same,  or  a  different  element,  by 
a  space  which  is  small  in  comparison  with  the  size  of  the 
atom. 

Applying  these  assumptions  more  particularly  to  carbon, 
on  each  atom  there  are  four  places,  or  parts,  having  different 
properties  from  the  rest  of  the  atom,  and  these  are  shown  to 
be  equidistant  from  each  other,  thus  determining  the  surface 
of  a  sphere.  Disregarding  for  the  time  the  shape  or  size  of 
these  valence-places,  they  are  made  up  of  matter,  and  thus 

1  Configuration  Organischer  Molekule.  Commissionsverlag  von  Bruno 
Leitholdt,  Leipzig. 


DEDUCTIONS  AND  SPECULATIONS.  175 

each  must  have  a  centre  of  gravity.  A  line  passing  through  a 
centre  of  gravity  of  a  valence-place,  and  that  of  the  atom 
itself,  is  called  an  axis ;  and  a  plane  perpendicular  to  an  axis, 
and  passing  through  the  centre  of  gravity  of  a  valence-place, 
is  called  a  valence-plane ;  and  for  simplicity  and  convenience, 
these  valence-planes  are  generally  referred  to  in  describing 
atomic  union,  rather  than  the  more  vague  valence-places. 
There  are  four  of  these  planes,  and  they  cut  each  other  in 
the  edges  of  a  regular  tetrahedron,  called  the  combination 
tetrahedron. 

Now,  two  carbon  atoms  may  approach  one  another  in  such  a 
way  that  their  valence-planes  shall  lie  parallel  to  one  another, 
but  it  will  be  impossible  for  them  to  touch,  since  the  whole 
of  the  valence-place  is  conceived  as  full  of  matter,  and  two 
bodies  cannot  occupy  the  same  place  at  the  same  time.  The 
nearer  the  valence-planes  are  to  one  another,  however,  the 
stronger  will  be  the  union.  It  is  conceivable  that  two  valence- 
planes  should  be  near  enough  together  to  constitute  union, 
and  yet  that  they  should  be  inclined  at  an  angle  to  one  an- 
other instead  of  being  parallel.  This  state  of  affairs  would 
correspond  to  the  bending  of  the  wires  in  ordinary  wooden 
models,  and  thus  is  offered  a  possible  explanation  of  von 
Baeyer's  "  strain  "  theory.  As  there  are  three  known  modes 
of  combination  between  two  carbon  atoms,  so  there  are  three 
main  positions  for  two  united  tetrahedrons. 

Here,  then,  by  an  entirely  different  mode  of  reasoning  he 
has  arrived  at  the  idea  of  forms  exactly  like  those  of  van't 
Hoff ;  but  in  Wunderlich's  conclusions,  single  linkage  is  rep- 
resented by  the  close  approach  of  two  tetrahedral  faces,  and 
triple  linkage  is  produced  by  bringing  three  faces  of  one  tetra- 
hedron simultaneously  as  near  as  possible  to  three  faces  of  the 
other;  i.  e.,  in  having  one  solid  angle  from  each  in  close 


176         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

proximity  to  that  of  the  other.  These  figures  are  reversed  in 
Wunderlich's  and  van't  Hoff's  schemes,  because  in  one  the 
seat  of  the  valence  is  placed  in  the  faces,  and  in  the  other  in 
the  solid  angles. 

"VYunderlich's  ideas  seem  to  be  in  complete  accordance  with 
the  facts  observed  in  double  and  triple  linkage.  In  the  case 
of  two  singly  linked  carbon  atoms,  the  two  combination  tetra- 
hedrons have  two  faces  parallel,  and  very  close  to  one  another. 
To  change  from  this  form  to  double  linkage,  these  tetrahedrons 
must  be  rotated  until  two  valence-places  come  as  near  together 
as  possible,  —  that  is,  until  the  two  tetrahedrons  have  an  edge 
in  common ;  but  now  the  combination-planes  are  no  longer 
parallel,  but  inclined  to  one  another,  and  so  the  strength  of 
each  unit  of  affinity  is  weakened.  Similarly,  in  changing  from 
double  to  triple  linkage,  a  still  further  rotation  must  take 
place,  until  three  valence-planes  of  one  carbon  atom  are 
equally  distant  from  three  valence-planes  of  the  other  carbon 
atom,  —  a  state  of  equilibrium  which  is  reached  when  the  two 
tetrahedrons  have  a  solid  angle  in  common ;  and  in  this  case 
the  combination-planes  are  still  farther  inclined  toward  one 
another,  and  therefore  the  strength  of  each  union  is  still  more 
decreased. 

It  may  be  mentioned  here  that  Skraup,  adopting  Wunder- 
lich's views,  has  offered  an  explanation  of  Wislicenus's  theory 
in  regard  to  the  addition  products  formed  from  doubly  linked 
carbon  compounds.  It  is  to  the  effect  that  double  linkage, 
while  forbidding  rotation,  admits  of  oscillations,  such  that  two 
valence-places  will  approach  each  other,  while  the  other  two 
draw  further  apart.  It  is  while  the  two  valence-places  are 
furthest  apart  that  the  union  is  most  easily  broken  by  the 
addition  of  new  radicals,  and  thus  it  is  that  the  two  added 
radicals  occupy  "corresponding"  positions. 


DEDUCTIONS  AND  SPECULATIONS.  177 

The  forms  which  have  been  described  for  singly,  doubly, 
and  triply  linked  carbon  atoms  have  been  called,  by  Wunder- 
lich,  the  "  main  positions ;  "  but  he  also  assumes  the  possibility 
of  union  in  intermediate  positions,  —  for  example,  cases  of 
single  linkage  in  which  the  combination-planes  are  not  exactly 
parallel,  and  illustrations  of  this  kind  he  finds  in  the  ring 
formations.  In  penta-methylene,  for  example,  the  mutually 
saturating  combination-planes  are  not  quite  parallel,  and  con- 
sequently the  centres  of  gravity  of  the  valence-places  are  not 
quite  so  near  one  another  as  they  are  in  C2H6.  Eeckoning  the 
amount  of  deviation  in  the  n  methylene  ring  from  the  formula, 

360°  —  n  (70°  31'  44") 
n 

it  is  seen  that  the  mutually  saturating  combination-planes 
from  di-  to  penta-methylene  rings  are  always  approaching 
nearer  to  one  another,  but  from  that  point  they  diverge 
again. 

Wunderlich  finds  the  benzene  formula  of  Kekule  more  in 
accordance  with  his  assumptions  than  that  of  Ladenburg,  and 
he  has  also  attempted  to  apply  his  theories  to  other  elements 
than  carbon,  notably  nitrogen  and  sulphur, 

It  has  been  seen  that  the  assumptions  which  Wunderlich 
makes,  which  are  of  importance  in  the  development  of  his 
theory,  are  that  the  carbon  atom  is  a  body  of  some  size  and 
shape,  and  that  it  has  four  portions  equidistant  from  each 
other,  which  are  distinguished  as  the  valence-places.  If  we 
assume  that  the  actual  shape  of  the  atom  is  that  of  a  tetrahe- 
dron, these  valence-places  are  located  in  the  centres  of  the  faces; 
but  the  atom  may  be  quite  as  satisfactorily  represented  as  a 
homogeneous  bullet,  from,  which  four  equally  large  segments 
have  been  cut  away,  so  that  each  of  the  four  resulting  faces  on 

12 


178         THE  DEVELOPMENT   OF  STEREO-CHEMISTRY. 

.  the  spherical  surface  has  the  same  position  with  relation  to  the 
other  three,  and  each  represents  a  valence-place. 

Auwers,  accepting  Wunderlich's  ideas  in  regard  to  the 
valence-places,  and  assuming  that  the  attraction  varies  in- 
versely as  the  square  of  the  distance  between  the  combination- 
planes  concerned,  has  calculated  the  ratio  of  the  strength  of 
the  double  link  to  that  of  the  triple,  and  finds  it  to  be  3.56  :  1, 
if  the  atoms  have  the  tetrahedral  shape.  If  the  atoms  are 
spherical,  this  ratio  becomes  1.65  :  I.1  His  calculations  also 
show  that  the  strength  of  the  double  and  triple  bonds  is 
greater  on  the  assumption  of  spherical  than  of  tetrahedral 
atoms,  and  though  they  cannot  be  directly  compared  with  the 
single  linkage,  they  can  never  be  two  and  three  times  as  great. 

Granting  that  stereo-chemistry  teaches  that  valence  acts  as 
a  located  or  directed  force,  there  is  still  room  for  a  difference 
of  opinion  as  to  whether  it  has  this  definite  location  inhe- 
rently, or  whether  it  is  induced  by  the  approach  of  other 
atoms;  and  also  as  to  the  general  nature  of  the  force  or 
property.  In  regard  to  the  first  question,  very  little  has  been 
definitely  stated  by  most  writers  on  the  subject,  but  it  seems 
to  be  tacitly  assumed  that  the  carbon  atom  has  four  places  or 
portions  which  are  different  from  the  rest  of  the  atom,  and  in 
which  the  attractive  force  is  located.  The  objection  to  be 
urged  against  this  is  that  it  makes  the  atom  a  more  compli- 
cated structure  than  its  practical  indivisibility  would  render 
probable.  If  the  differentiation  in  these  four  particular  places 
is  due  either  to  a  different  kind  of  matter,  or  to  a  different 
kind  of  motion,  it  would  appear  to  have  a  tendency  to  destroy 
the  unity  of  the  atom.  If  due  to  a  different  amount  of  con- 
densation of  the  same  kind  of  matter  as  that  composing  the 
rest  of  the  atom,  it  is  difficult  to  conceive  of  its  being  inherent, 
1  Entwickelung  der  Stereo-chemie,  pp.  28-34. 


DEDUCTIONS  AND  SPECULATIONS. 


179 


though,  it  is  quite  conceivable  that  this  greater  condensation 
might  be  induced  in  certain  portions  on  the  near  approach  of 
other  atoms.  This  would  suit  perfectly  Wunderlich's  concep- 
tion of  a  sphere  with  four  equidistant  segments  removed,  — 
the  removed  segments  being  replaced  in  this  conception  by  a 
more  attenuated  form  of  matter ;  but  the  difficulty  is  to  recon- 
cile any  such  idea  with  the  impossibility  of  free  rotation  in 
doubly  linked  compounds.  That  one  portion  of  stereo-chemical 
theory  seems  to  necessitate  the  conception  of  the  carbon  atom 
as  existing  with  four  definite  and  inherent  valence-places. 

Nevertheless,  Werner l  does  not  consider  the  results  of 
stereo-chemistry  incompatible  with  the  idea  of  valence  as  an 
attractive  force,  working  symmetrically  from  the  centre  to  the 
surface  of  the  atom,  conceived  as  spherical  and  homogeneous. 
On  this  supposition,  the  four  radicals,  combined  with  the  car- 
bon, will  lie  in  the  angles  of  a  tetrahedron,  because  in  this 
position  the  greatest  exchange  of  affinity  between  the  atoms 
can  take  place.  According  to  this,  there  are  no  separate 
valencies,  and  no  real  double  or  triple  linkage.  The  system, 
a  b  C  Ca  b,  will  be  stable  in  two  different  configurations, 
Figs.  (1)  and  (2)  :  — 


Beitrage  zur  Theorie  der  Affinitat  und  Valenz. 


180         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

Two  portions  of  the  total  amount  of  attractive  power  in 
each  carbon  atom  are  used  up  in  holding  a  and  5,  and  two 
portions  are  left  to  hold  the  other  carbon  atom.  According 
to  Werner,  Xi  must  be  thus  used  under  all  circumstances,  and 
does  not  hinder  free  rotation;  but  the  portions  called  x2  are 
only  used  to  hold  the  two  carbon  atoms  together  under  special 
circumstances,  and  therefore  these  are  the  portions  which 
hinder  the  free  rotation.  This  xz  is  small  in  proportion  to  the 
total  amount  of  attractive  force,  and  can  be  weakened  by  heat, 
and  in  other  ways,  until  it  no  more  opposes  the  effort  of  the 
groups  a  and  b  to  assume  the  most  favorable  configuration. 
Such  is  Werner's  theory ;  but  it  is  far  from  being  entirely 
satisfactory,  since  he  does  not  make  it  clear  in  what  way  this 
xz  limits  the  rotation. 

Van't  Hoff,  in  his  "  Ansichten  ueber  Organische  Chemie,"  l 
regards  the  chemical  union  of  atoms  as  a  consequence  of  gravi- 
tation. If  the  form  of  an  atom  is  not  spherical,  he  shows  that 
the  amount  of  attraction  of  that  atom  for  others  will  be 
marked  by  a  certain  number  of  maximum  points  on  the  surface 
of  the  atom,  which  maxima  depend  on  the  form  of  the  atom, 
and  may  have  different  values.  The  number  of  these  maxima 
is  considered^  expressing  the  valence  of  the  atom.  Assum- 
ing that  the  form  of  the  atom  changes  through  its  vibratory 
motions,  then  the  valency  of  the  atom  may  vary  with  varia- 
tions in  the  state  of  motion  of  the  atom,  and  these  varia- 
tions will  be  conditioned  by  temperature,  nearness  to  other 
atoms,  etc. 

Another  suggestion  in  regard  to  the  nature  of  valence  has 
been  made  by  Sachse,2  who  has  endeavored  to  explain  the 
phenomenon  on  the  supposition  that  the  atoms  are  made  up 
of  little  parts  similar  to  magnets. 

i  Part  I.  pp.  2-5.  2  Zeit.  ph.  Chem.,  xi.,  185. 


DEDUCTIONS  AND  SPECULATIONS.  181 

The  idea  that  the  chemical  union  of  atoms  is  a  phenomenon 
of  an  electrical  nature  is  an  old  one.  Berzelius  advanced  the 
idea  that  two  atoms  of  different  elements  coming  into  contact 
excited  each  other  electrically,  like  the  metals  in  Volta's  ex- 
periment ;  but  he  supposed  that  the  quantity  of  electricity 
collected  at  the  point  of  union  of  two  atoms  depended  on  their 
chemical  affinity  for  each  other.  This  Faraday  proved  to  be  a 
mistake,  showing  that,  so  far  as  this  electricity  came  forth  in 
electrolytic  decomposition,  its  quantity  did  not  at  all  depend 
on  the  degree  of  affinity,  but  was  rather  connected  with  the 
valence.  Thus  he  found  that,  using  currents  of  constant  in- 
tensity in  decomposing  different  compounds,  the  amount  of 
decomposition  in  cells  containing  different  electrolytes  is  ex- 
actly proportional  to  the  chemical  equivalent  of  the  elements 
which  were  either  separated  or  converted  into  new  com- 
pounds. 

Helmholtz,1  who,  in  1881,  directed  the  attention  of  chemists 
anew  to  Faraday's  work,  says :  "  If  we  accept  the  hypothesis 
that  the  elementary  substances  are  composed  of  atoms,  we 
cannot  avoid  concluding  that  electricity  is  also  divided  into 
definite  elementary  portions  which  behave  like  atoms  of  elec- 
tricity. The  same  atom  can  be  charged  in  different  compounds 
with  positive  or  negative  electricity.  Faraday  believed  that 
the  forces  termed  chemical  affinity  and  electricity  are  one  and 
the  same.  I  think  the  facts  leave  no  doubt  that  the  very 
mightiest  among  the  chemical  forces  are  of  electrical  origin. 
The  atoms  cling  to  their  electric  charges,  and  opposite  electric 
charges  cling  to  each  other ;  but  I  do  not  suppose  that  other 
molecular  forces  are  excluded  working  directly  from  atom  to 
atom." 

In  the  new  edition  of  Watts's  "  Dictionary  of  Chemistry," 
i  J.  Chem.  Soc.  Trans,  for  1881. 


182         THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 

in  the  article  on  Isornerism,  Armstrong  says  :  "  The  deduction 
from  Faraday's  law  of  electrolysis,  that  definite,  as  it  were 
atomic,  charges  of  electricity  are  associated  with  the  atoms  of 
matter  ...  is  the  only  approach  yet  made  to  a  theory  of 
valency ;  but  hitherto  chemists  have  avoided  the  subject  from 
this  point  of  view." 

In  1888,  however,  Victor  Meyer  and  Biecke1  published  a 
paper  containing  an  electrical  explanation  of  valence,  based 
partially,  at  least,  on  studies  in  stereo-chemistry.  Eiecke  had 
approached  the  subject  from  the  physical  standpoint,  and,  by 
his  studies  in  pyro-electricity,  had  coine  to  the  conclusion  that 
each  valency  is  caused  by  a  certain  combination  of  two  oppo- 
sitely electrified  particles,  and  Meyer  had  reached  a  similar 
conclusion  from  his  chemical  study  of  the  benzil  dioximes  and 
other  bodies.  The  latter,  from  the  study  of  certain  cases  of 
isomerism  already  mentioned,  draws  the  conclusion  that  in 
case  of  single  linkage  between  two  carbon  atoms,  there  may 
be  two  different  kinds  of  combination,  —  one  in  which  free 
rotation  in  reverse  directions  is  possible,  and  another  in  which 
it  is  impossible. 

As  this  is  as  far  as  the  chemical  outlook  can  carry  the  sub- 
ject, it  is  next  attacked  from  the  physical  side.  The  facts  of 
frictional  and  galvanic  electricity  lead  to  the  assumption  that 
the  chemical  elements  are  not  simply  combined  among  each 
other  by  the  working  of  affinity,  but  that  effects  of  the  same 
character  also  result  between  the  chemical  elements  and  the 
electric  fluids.  The  phenomena  of  pyro-electricity  lead  to  the 
assumption  that  the  molecules  of  the  pyro-electric  crystal  are 
combined  with  a  system  of  electric  poles  which  possess  a  fixed 
position  with  relation  to  each  other,  and  to  the  molecule  itself ; 
and  it  is  but  a  step  farther  to  suppose  that  the  atoms  may  be 

i  Ber.  21,  946. 


DEDUCTIONS  AND   SPECULATIONS.  183 

surrounded  by  similar  systems  of  electrical  poles.  It  has  al- 
ready been  pointed  out  that  some  deductions  in  regard  to  the 
connection  of  electricity  with  atoms  may  also  be  made  from 
Faraday's  law  of  definite  electrolytic  action,  which  Meyer 
formulates  as  follows :  "If  a  current  passes  for  the  same 
length  of  time  through  several  electrolytes,  in  each  of  these 
the  same  number  of  valencies  is  loosed."  This  makes  a  very 
close  connection  between  atoms  and  electricity,  and  especially 
between  the  valence  of  the  atom  and  its  electric  charge,  since 
'the  amount  of  the  charge  seems  to  be  the  same  in  all  atoms  of 
monad  elements,  double  that  amount  in  dyads,  etc. 

Such  considerations  as  the  preceding  have  led  Meyer  and 
Riecke  to  the  following  assumptions  :  The  carbon  atom  is 
surrounded  with  an  ether  envelope,  which  is  spherical,  like 
the  atom  itself ;  and  its  diameter  is  several  times  larger  than 
that  of  the  atom.  The  atom  itself  is  considered  as  the  bearer 
of  the  specific  affinity,  and  the  surface  of  the  envelope  as  the 
seat  of  the  valence.  Each  valency  is  conceived  as  produced 
by  the  existence  of  two  oppositely  electrified  poles,  repre- 
sented as  situated  at  the  end  point  of  a  straight  line,  short  in 
comparison  with  the  diameter  of  the  ether  envelope.  Such  a 
system  of  two  electric  poles  they  call  a  di-pole,  and  carbon 
has  four  such  di-poles.  The  middle  point  of  each  is  conceived 
as  bound  to  the  surface  of  the  ether  envelope,  but  freely  mov- 
able in  this  ;  and  the  di-poles  are  freely  rotatable  around  their 
middle  points.  A  farther  assumption  is  that  the  carbon  atom 
possesses  a  greater  attraction  for  the  positive  than  for  the 
negative  electricity,  so  that  all  of  the  valencies  will  turn  their 
positive  poles  toward  the  carbon  atom ;  and  it  is  also  assumed 
that  the  positive  poles  of  the  valencies  have  somewhat  more 
strength  than  the  negative.  The  di-poles  of  one  and  the  same 
carbon  atom  repel  each  other  so  that  in  an  isolated  carbon 


184         THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 


atom  there  will  be  equilibrium  if  the  valencies  are  found  in 
the  angles  of  a  regular  tetrahedron ;  but  they  can  be  deflected 
from  this  position. 

When  two  carbon  atoms  come  so  near  one  another  that  two 
di-poles  are  in  immediate  juxtaposition,  union  takes  place, 
and  in  consequence  of  the  repulsion  between  like  and  the 
attraction  between  unlike  poles,  these  so  arrange  themselves 
that  the  positive  poles  of  one  lie  next  the  negative  poles  of 
the  other,  and  vice  versa ;  but  since  the  di-poles  are  rotatable, 
two  different  positions  are  possible  with  single  linkage,  as 
shown  in  the  following  figures :  — 


(2) 


In  (1)  the  atoms  are  freely  rotatable  around  the  axis  joining 
the  two  carbon  atoms ;  but  in  (2)  they  are  not.  By  far  the 
more  common  kind  of  linkage  is  that  represented  in  (1),  — 
the  kind  shown  in  (2)  being  only  found  in  cases  where  the 
connected  radicals  are  of  very  nearly  the  same  electrical 
character,  so  that  there  is  no  strong  attraction  between 
them. 

Double  linkage  is  represented,  according  to  their  theory,  by 
the  following  figure :  — 


DEDUCTIONS  AND  SPECULATIONS. 


185 


Auwers  lias  suggested  a  slight  modification  of  these  ideas  to 
account  for  the  fact  that  the  double  linkage  is  not  twice  as 
strong  as  the  single.  He  says  that  if  the  ether  envelope  be 
considered  as  only  a  little  larger  than  the  atom  itself,  the 
di-poles  would  then  lie  nearer  the  surface  of  the  atom ;  and  in 
case  of  double  and  triple  linkage,  the  di-poles  of  different 
atoms  could  no  more  lie  in  immediate  juxtaposition,  but  must 
act  at  a  certain  distance  from  each  other. 

These  conceptions  are  somewhat  difficult  to  grasp  in  full 
detail,  and,  until  a  larger  number  of  well- authenticated  cases 
of  isomerism  are  found  to  require  such  an  explanation,  it 
seems  hardly  necessary  to  adopt  any  theory  to  explain  limited 
rotation  in  singly  linked  carbon  compounds.  The  general  idea, 
however,  of  the  spherical  atom,  as  charged  with  four  units  of 
electricity,  evenly  distributed  either  on  its  surface  or  that  of  its 
ether  envelope,  seems  to  be  in  good  accordance  with  the  facts 
observed,  and  recommends  itself  highly  to  our  judgment.  -It 
offers  an  explanation  of  the  located  valence-places,  and  is  in 
accordance  with,  or  rather  a  deduction  from,  Faraday's  law. 
In  the  present  state  of  knowledge  regarding  the  nature  of 
electricity,  any  speculations  of  this  kind  must  be  very  crude 
and  vague ;  but  the  indications  of  a  close  relationship  between 
chemical  attraction  and  electricity  are  strong  enough  certainly 


186        THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 

to  make  it  seem  possible  that  valence  may  properly  be  ex- 
plained in  some  such  way  as  that  pointed  out  by  Meyer  and 
Riecke. 

Helmholtz,  some  time  since,  to  account  for  the  difference  in 
intensity  of  attractive  power  in  different  elements,  suggested 
that  their  electric  charges  might  be  held  to  the  atoms  with 
varying  degrees  of  attractive  force.  Is  it  not  possible,  then, 
that  in  the  same  atom,  considering  its  electric  charge  as  made 
up  of  definite,  located  units,  some  may  be  held  to  the  atom 
with  less  force  than  others,  and  thus  some  of  these  units  be 
always  active,  while  others  are  sometimes  active  and  some- 
times dormant,  thus  accounting  for  varying  valence  ? 

None  of  the  theories  thus  far  mentioned  take  into  account  the 
subject  of  varying  valence ;  and  perhaps  this  is  not  necessary 
in  the  case  of  carbon,  since  carbon  monoxide  is  the  only  com- 
pound which  is  absolutely  inexplicable,  assuming  the  valence 
of  carbon  to  be  four,  though  Nef 1  makes  a  strong  argument  in 
favor  of  the  valence  of  carbon  as  two  in  the  iso-cyanides.  It 
has  been  suggested  that  the  existence  of  carbon  monoxide  may 
be  explained  equally  well  by  the  assumption  of  the  tetra- 
valence  of  oxygen  as  by  the  bivalence  of  carbon ;  and  Friedel,2 
in  support  of  the  former  view,  gives  arguments  for  the  tetra- 
valence  of  oxygen  in  (CH3)20.  HC1,  and  also  suggests  that  it 
may  be  to  the  two  extra  bonds  of  oxygen  that  the  existence 
of  substances  containing  water  of  crystallization  may  be  due. 
Even  granting,  however,  the  invariability  of  the  valence  of 
carbon,  any  explanation  of  this  property  is  of  little  value  un- 
less it  can  be  applied  also  to  other  atoms,  and  therefore  should 
not  be  of  such  a  nature  as  to  preclude  the  possibility  of  vary- 
ing valence. 

1  Lieb.  Ann.  d.  Chem.  270,  267. 

2  Bull.  Soc.  Chim.  24,  160,  241. 


DEDUCTIONS  AND   SPECULATIONS.  187 

The  principal  deductions  and  speculations  which  have  re- 
sulted either  directly  or  indirectly  from  the  study  of  stereo- 
chemistry have  been  briefly  reviewed  in  the  foregoing.  These 
results  may  be  summed  up  as  follows  :  — 

An  atom  must  be  considered  either  as  a  material  point  or  a 
body  of  finite  expansion  in  space.  The  teachings  of  stereo- 
chemistry are  against  the  former,  and  in  favor  of  the  latter 
hypothesis.  Facts  seem  to  indicate  that  the  atom  has  a  fixed 
and  definite  form ;  this  form  must  be  either  that  of  a  sphere 
or  a  symmetrical  polyhedron,  and  though  stereo-chemistry  offers 
no  definite  teaching  on  this  point,  the  spherical  form  seems 
simpler  and,  for  reasons  before  given,  more  probable. 

If  stereo-chemistry  teaches  anything  in  regard  to  the  carbon 
atom,  it  is  that  its  attractive  force  is  concentrated  in  four 
places,  symmetrically  arranged  with  reference  to  each  other 
and  to  the  centre  of  the  atom.  Some  investigators  regard  the 
valence  as  a  force  which  can  be  exerted  only  in  certain  definite 
directions,  and  symbolize  this  idea  by  representing  the  carbon 
atom  as  a  bullet,  from  which  four  rods  proceed  in  the  direc- 
tions of  the  angles  of  a  tetrahedron  j  others  think  of  valence 
as  caused  by  the  existence  of  four  symmetrically  arranged 
portions,  which  are  qualitatively  different  from  the  rest  of  the 
atom,  and  which  are  called  valence-places.  The  nature  of 
valence,  considered  as  a  force,  is  by  some  supposed  to  be  a 
particular  form  of  universal  gravitation,  and  by  others  to  be 
of  an  electrical  or  magnetic  character. 

Some  chemists  urge  that  the  valence  of  carbon  cannot  be 
a  priori  divided  into  four  portions  in  the  atom,  but  must  be  a 
unified  whole  until  the  approach  of  other  atoms  separates  it 
into  portions.  This  conception  is  certainly  the  one  which  is 
most  in  harmony  with  our  preconceived  ideas  of  atoms  and 
attractive  forces  ;  but  the  teachings  of  stereo-chemistry  do  not 


188        THE  DEVELOPMENT  OF  STEREO-CHEMISTRY. 

justify  this  conclusion.  If  we  considered  only  compounds 
containing  a  single  carbon  atom,  we  should  have  little  diffi- 
culty in  reconciling  the  facts  with  the  theory  either  of  inherent 
or  of  induced  valence-places ;  but  the  trouble  comes  with  the 
consideration  of  the  so-called  doubly  linked  compounds,  in 
which  we  must  account  for  the  impossibility  of  rotation  in 
opposite  directions  of  the  two  carbon  atoms,  and  for  the  fact 
that  the  '/bonds"  cannot  be  considered  as  units  in  the  sense 
of  a  double  bond  being  twice  as  strong  as  a  single  one.  Of 
these,  the  former  seems  the  more  serious  difficulty.  In  the 
molecule  C2H4,  it  is  conceivable  that  a  certain  condition  may 
be  induced  in  the  portions  of  the  carbon  atom  lying  next  the 
hydrogen,  which  should  differentiate  them  from  the  rest  of 
the  atom ;  but  a  similar  condition  would  naturally  be  induced 
in  the  adjacent  portions  of  the  two  carbon  atoms,  and  we  find 
nothing  in  this  to  prevent  rotation.  We  can  only  conceive  of 
the  rotation  as  hindered  in  case  the  carbon  atoms  are  held 
together  at  two  points,  a  a)  and  b  b'j  so  that  the  rotation  of  one 
atom  alone  would  break  this  connection. 


The  reason  for  the  double  linkage  not  being  twice  as  strong 
as  the  single  is  also  shown  in  such  a  conception  as  this,  for  in 


DEDUCTIONS  AND  SPECULATIONS.  1 89 

the  above  illustration  the  valence-places  do  not  come  as  closely 
in  contact  with  each  other  as  in  single  linkage. 

Granting,  then,  the  existence  of  inherent  valence-places, 
there  is  still  diversity  of  opinion  as  to  whether  these  are 
caused  by  a  qualitative  difference  of  matter  at  these  points,  or 
whether  they  are  the  results  of  a  polar  condition  either  in  the 
atom  itself  or  its  ether  envelope ;  and  in  regard  to  this  point 
stereo-chemistry  has  nothing  to  say.  Having  thrown  down  the 
postulate  of  the  existence  of  valence-places,  stereo-chemistry 
withdraws,  having  apparently  no  facts  to  offer  in  explanation 
of  the  cause  and  nature  of  such  places.  These  subjects  seem 
at  present  to  be  left  largely  to  the  domain  of  pure  speculation, 
though  there  is  an  undoubted  and  proved  connection  between 
electricity  and  valence  which  cannot  be  overlooked  in  any 
explanation  of  the  latter. 

It  is  plain,  then,  that  stereo-chemistry  offers  no  distinct  and 
definite  representation  of  an  atom.  It  only  emphasizes  cer- 
tain attributes  of  the  atom,  and  has  already  been  very  fruitful 
in  stimulating  speculations  concerning  atomic  structure  and 
valence.  Whether  any  one  of  the  theories  now  before  the 
public,  or  one  yet  to  be  evolved,  will  ever  receive  experi- 
mental verification  enough  to  be  yielded  universal  acceptance, 
and  thus  give  a  definite  conception  of  the  atom  or  not,  time 
alone  can  tell ;  but  the  difficulty  of  the  problem  rests  in  its 
simplicity,  and  perhaps  none  of  the  solutions  yet  offered  are 
simple  enough  to  be  the  true  one. 


190        THE   DEVELOPMENT  OF  STEREO-CHEMISTRY. 


LIST   OF  BOOKS   CONSULTED.1 

THE  following  is  a  list  of  the  principal  books  and  articles 
on  the  general  subject  of  stereo-chemistry  which  have  been 
freely  consulted  in  the  preparation  of  this  volume :  — 

Auwers.     Die  Entwickelung  der  Stereo-chemie. 

Le  Bel.     Bulletin  de  la  Societe  Chimique  [2],  22,  337. 

Eiloart.     Guide  to  Stereo-chemistry. 

Hantzsch.     Grundriss  der  Stereo-chemie. 

Van't  Hoff.     Die  Lagerung  der  Atome  im  Raume.     Herrmann. 

Van't  Hoff.     Dix  Annees  dans  Phistoire  d'une  Thdorie. 

Van't  Hoff's  Chemistry  in  Space.  Translated  and  edited  by  J.  E. 
Marsh. 

Van't  Hoff.     Bulletin  de  la  Socidte  Chimique  [2],  23,  295. 

Meyer,  R.     Jahrbuch  der  Chemie.     1891  and  1892. 

Meyer,  V.  Ergebnisse  und  Ziele  der  stereo-chemischen  Forschung. 
Berichte  der  deutschen  chemischen  Gesellschaft,  23,  567. 

Pasteur.  Ueber  die  Asymmetric  bei  natiirlich  vorkommenden  orga- 
nischen  Verbindungen.  Uebersetzt  von  Ladenburg. 

Warder.  Proceedings  of  the  American  Association  for  the  Advance- 
ment of  Science,  vol.  39,  p.  Xll. 

Wislicenus.  Ueber  die  raumliche  Anordnung  der  Atome  in  organi- 
schen  Molekiilen  und  ihre  Bestimmung  in  geometrisch-isomeren 
ungesattigten  Verbiudungen. 

1  Bischoff's  Handbuch  der  Stereo-chemie  has  been  completed  since  the 
above  list  was  made  out. 


PERIODICALS  REFERRED  TO.  191 


PERIODICALS   TO   WHICH   REFERENCE 
HAS   BEEN   MADE. 

Am.  Chem.  J.  =  American  Chemical  Journal. 

Annales  de  Chim.  et  de  Phys. 

Ber.  =  Berichte  der  deutschen  chemischen  Gesellschaft. 

Bull.  Soc.  Chim.  =  Bulletin  de  la  Societe  Chimique  de  Paris. 

Compt.  rend.  =  Comptes  rendus. 

J.  Chem.  Soc,  =  Journal  of  the  Chemical  Society. 

J.  pr.  Chem.  =  Journal  fiir  praktische  Chemie. 

Lieb.  Ann.  d.  Chem.  =  Liebig's  Annalen  der  Chemie. 

Monatshefte  fiir  Chemie. 

Zeit.  ph.  Chem.  =  Zeitschrift  fiir  physikalische  Chemie. 

Zeitschrift  fiir  Krystallographie. 


LD  2 1-100  r 


